This theory has several elements to it, but boils down to the idea that Doran Martell’s master plan could not have been simply to arrange a marriage between Arianne and Viserys, then turn around and send Quentyn on his doomed quest to marry Daenerys. Doran Martell is too sophisticated a schemer, is conveying something more to Arianne at the end of A Dance with Dragons than this, and was admittedly working with Oberyn on something more than just a marriage contract than the mere marriage contract version of his plan implies. Doran and Oberyn have stated their goals for their plan: justice, advancement, and revenge. Once the specifics of their plan are related to these goals, the Martell Master Plan comes into sharp focus becoming much more grandiose and important than is commonly understood.
The easiest part to discern is the revenge element. Doran and Oberyn hold Tywin Lannister, Amory Lorch, and Gregor Clegane responsible for the murder and rape of their sister, Elia, and her two children (Rhaenys by Lorch, Aegon and Elia by Gregor). Lorch is killed by the Brave Companions, Gregor by Oberyn, later transformed into a zombie/Frankenstein’s monster by Qyburn, a former member of the Brave Companions. To achieve Doran’s stated goal of removing everything Tywin holds dear before killing him, Jaime Lannister is maimed by the Brave Companions, thus crippling Tywin’s favorite and hoped for legacy, and Tywin himself appears to have suffered from Widow’s blood poisoning, which operates by constricting the bowels and caused his corpse to irretrievably smell prior to his burial on public display no less.
As for how the Martells poisoned Tywin, the Alt Shift X video is quite correct in stating that Oberyn would have had no chance to place anything of the kind in Tywin’s food or drink, not the least of which is Oberyn would be suspected of doing so with his hatred of Tywin and his reputation as a poisoner. That Oberyn was seeking a public trial by combat with the Mountain was no secret. Therefore, he was expecting to be put on trial and to have to fight the Mountain (the obvious choice as the Lannister’s champion) for having poisoned Tywin. However, the actual poisoner was someone who had access to Tywin’s food and drink, who would not be suspected, and came into contact with Tywin just before Tywin started showing signs of the poisoning. Only one person fits all these characteristics and seems to have been moving into that position before the Battle of the Greenforks in Tywin’s camp: Shae, whose looks are Dornish, is in Tywin’s bed, and has gone over to Tywin just after Oberyn’s arrival and Tyrion’s looting of Pycelle’s collection of drugs.
The Brave Companions’ sacking of churches and killing of septons and septas supposedly while serving the Lannister cause in the War of the Five Kings has also caused the Sparrow Movement, which led directly to the rise of the fundamentalist High Sparrow. The High Sparrow has had Cersei undergo the walk of shame and is presently arranging for the return of fundamentalist Faith of the Seven practices through the newly revived Faith Militant. The rejection of polygamy and incest by the Faith of the Seven means that Cersei’s remaining children – Tommen and Myrcella – are in jeopardy as are the Targaryen claimants. House Lannister – Tywin’s pride and joy – is about to be brought very low. Not incidentally, the rise of the fundamentalist Sparrows makes Stannis Baratheon an unsuitable candidate for the throne because of his endorsement of R’hllor. This leaves a power gap at the center of Westeros.
Viewed in this light, this makes The Brave Companions the as yet unnamed sellsword company that Oberyn founded after leaving the Second Sons. But, what remains are the ultimate goal of Oberyn and Doran’s scheming, which, as one can tell by the birth order of Oberyn’s daughters, began well before Robert’s Rebellion and the subsequent murder and rape of Elia and murder of her children. The only possible goal is not the ascendance of the Targaryens with a Martell as a spouse, but the elevation of Doran’s heir – Arianne – to the Iron Throne, the entire point of the game of thrones.
The key to this grand maneuver lies with the four books Doran placed in Arianne’s prison cell: a history of Westeros, the book of the Faith of the Seven – The Seven Pointed Star, a book of Dornish Law, and a book on dragons. The Seven Pointed Star recounts the Faith’s opposition to both incest and polygamy. The book on Dornish Law contains Dorne’s unique inheritance laws that do not privilege males over females. The book on dragons would be useful for either ensnaring or killing a dragon or dragons, whichever is needed. The history of Westeros would inform the reader of the fact that Rhaenys, sister-wife to Aegon the Conqueror, was married second to her older sister, Visenya, was independently crowned, and was downed in Dorne when her dragon, Mertaxes, was felled by a scorpion bolt. Shortly after this devastating blow, Prince Nymor of Dorne delivered a letter to Aegon the Conqueror that caused him to grip it so hard his hands bled, proceeded to fly on Balerion’s back to Dragonstone, then return and sign a peace with Dorne after having destroyed the letter.
While we do not know the content of this letter – the Red Letter if you will, we can guess at its contents in light of the Martell plan to claim the Iron Throne. Under strict observance of the Faith of the Seven, Rhaenys’ marriage to Aegon, thus any of the product of that union from whom all Targaryens trace their heritage, was invalid as both a bigamous marriage and an incestuous union. Thus, Rhaenys’ subsequent marriage, and any offspring she bore would be her only legitimate children. Also, under Dornish Law, because she was crowned independently, her children would have a claim to the throne. A letter confirming these events including a) Rhaenys’ marriage to Prince Nymor, b) her bearing a child with a better claim to the throne of Westeros, c) is happily conveying all she knows about dragons, and d) has taken dragon eggs as a dowry from Dragonstone might well cause Aegon, who preferred Rhaenys over Visenya, to bloodily grip it and cease war against Dorne. Thus, once the Martells trick Cersei into establishing Dornish Law for all of Westeros (Cersei wants to inherit Casterly Rock over Jaime and Tyrion as the eldest of the three.), Arianne Martell becomes the only legitimate claimant to the Iron Throne.
In light of these observations, the Quentyn mission becomes, not the sum total of the plan, but a diversion that was meant to fail. Surrounded by the servants of the Yronwoods – a rival house to the Martells, the equivalent of Dorne’s Boltons, not given anything in the way of substantial resources, and meant to fulfill laughable goals even had Quentyn been handsome, Quentyn’s quest was Doran’s attempt to satisfy the other major group of conspirators – the Varys/Illyrio plot to put (fake) Aegon on the throne. Note that the Windblown (who serve Illyrio) and Second Sons (who serve the Martells as Oberyn’s former sellsword company) as well as Quentyn’s own companions thwart his efforts. This is aside from Quentyn’s near fatal attempt to secure Viserion, who, not so incidentally, seems less than hostile to him. It is only the defect in Quentyn’s oil-soaked whip that causes it and Quentyn’s clothing to catch flame near Rhaegal’s hot breath. Quentyn is alive, but badly burned. Unfortunate, he is a victim of his father’s terrible secrecy that also causes the bad blood between Doran and his true heir, Arianne. He was never meant to succeed and certainly not meant to try and steal a dragon.
The only aspect of the Martell Master Plan that has failed to this point is the attempt to birth and secure dragons. The first attempt was Mirri Maz Duur. Mirri’s qualifications – an apprentice to Marwyn (who worked closely with Oberyn and Qyburn, now Sarella/Alleras) at the Citadel, a priestess of Asshai, and so on – mark her as a Martell plant. Her location in a Lhazareen village in the Dothraki’s path is a set up. Her actions that turned out so badly that Daenerys burned her on Drogo’s funeral pyre were failed attempts to heal. Drogo caused her poultice to fail when he removed it to put mud in the wound, drank alcohol, and took milk of the poppy in defiance of Mirri’s instructions. Daenerys’ miscarriage resulted from Jorah bringing Daenerys into the tent in violation of Mirri’s warnings. Finally, it was not the blood sacrifice that caused the dragon eggs to hatch, but Daenerys having dragon egg hatching genes and being around the eggs for a decent period of time. The eggs were warming prior to the funeral pyre and had begun to hatch before the horse and Mirri were sacrificed. Also, Mirri’s ululating was protecting her from the flame, likely something she learned in Asshai, which specializes in fire magic. Daenerys’ entering the pyre stole the immunity, causing Mirri to burn, and Daenerys to emerge relatively unscathed. The Cinnamon Wind, Marwyn’s ship as we learned from Sam’s chapter in A Dance with Dragons, docked in Qarth, was likely Mirri’s transport.
The second attempt to steal dragons came from Brown Ben Plumm’s machinations in Slavers’ Bay. Plumm, with Valyrian blood it seems, was liked by the dragons and comfortable around them. It was only Daenerys locking away Viserion and Rhaegal in a maze in the base of the Harpy’s pyramid that thwarted this aspect of the plan. (It is certainly possible that Viserion is with Quentyn in one of the pyramids. Unlike Rhaegal who likes nesting outside, Viserion behaves like a bat and prefers living inside his pyramid.)
Rather than being a sidelight or a meditation on the silliness of quests, the Martell Master Plan has worked and is working in spectacular fashion. Tywin Lannister and his men have suffered terribly for their crimes. Marwyn is on his way to become Daenerys’ maester. If they have not secured a dragon, they are in possession of a book that will be more than enough to deal with one or more if that is necessary. The newly armed and fanatical Faith of the Seven are primed to be key allies. Doran’s nieces, Nym and Tyene, are on the Small Council and with the High Sparrow, respectively. Arianne and Obara are in a place where they can aid the Blackfyre/Aegon cause and threaten Oldtown. The Martells are in position to take over Westeros. Instead of being on the sidelines and unimpressive, they may well be the winners of the game of thrones.
Note: The vast majority of the foregoing is from Preston Jacobs’ “Dornish Master Plan”, “Deeper Dorne”, and “Quentyn Is Alive” YouTube videos with the exception of the material on the Red Letter, which is largely, but not wholly, derived from Sean T. Collins among others, and Shae’s role, which is my conclusion and that of Cupboardstain on Reddit.