How is this a good theory? Why are Arys and Tywin wife swapping? Are they part of a swingers club? And why is Rhaella pretending to be Joanna (pretending to who exactly?) And Joanna pretending to be Rhaella?
If Rhaella is Tyrions mother, then is she the one who died giving birth? If so, did they bury her under Joanna's name?
Sorry but this does not sound like a good theory at all (imo) and really unnecessarily complicated.
Edit: Did you mean they are the same person?
That makes even less sense.
Crazy theory Joanna Lannister and Rahella Targaryen swap identities.
Why?
Rahella Targaryen hated her brother Arys and didn't want to marry him so she concocted a plan along with one of her ladies in waiting Joanna Lannister to swap lives. As both were always together and had an almost identical appearance as they grew up both had the opportunity and motivation to swap lives.
Who knew, did they have the motivation and opportunity to pull off this ruse.
Arys Targaryen- Arys was smitten with Joanna Lannister and is rumored to have deflowered her. By having Joanna swap Identities with a person whom he loved Aryes was looking forward to a long and happy marrige with someone got along with as opposed to Rahella who he was said to despise. This deal was something Arys would later come to regret as he was faced with failed births. This would ultimately cause the bad blood between Arys and Tywin and lead to Arys joking about taking liberties with the bride at Tywin and Joanna Lannister (Actually Rahella Targaryen)'s wedding as he was wistfully wondering about what could have been and then the eventual rape of Joanna Lannister (Rahella) that led to the birth of Tyrion Lannister.
Tywin Lannister
Tywin Lannister was one of the most ruthless men in the seven kingdoms and hellbent on restoring his family's honor after the reign of his father Tytos. In the early days he was the hand of King Arys before a falling out and was said to be the most powerful man in the kingdoms as hand of the king.
Contrary to his known nature. Tywin Lannister (a man obsessed with status and power above all else) supposedly married his cousin for love. This is supported by the fact that he smiled on his wedding day. This doesn't make much sense. Tywin was a player of the game of thrones and ambitious to a fault. He would be just the type of person to secretly marry a princess and then try and join house Lannister with House Targaryen as he does by offering Cersi as a match for Prince Rhaegar before being turned down.
This also somewhat explains Tywins treatment of Tyrion and especially his first wife Tysha where Tywin had her raped by his men and then paid for her service. This doesn't sound like the behavior of a man who would marry for love. Yet it starts to make sense when you see it as what it is. Retribution against Arys son for the rape of Tywin's wife whom he did actually love.
Rahella Targaryen
Hated and despised her brother and quickly fell in love with the dashing and brilliant hand of the king. Tywin was also smitten to the point that he even is said to have smiled at his wedding. Rahella wanted to get away from her brother and developed a plan to do so, swapping lives with her ambitious lady in waiting Joanna Lannister who had previously slept with Arys. Given the likely similar apperance of Joanna and Rahella this would have been easy to pull off.
Joanna Lannister
Was just as ambitious as Tywin and wanted to be Queen. This was her way to achieve that.
Circumstantial Evidence
Foreshadowing.
GRRM likes to use history/past events in his world to inform/foreshadow the present. In this case the two key events are "The Dance of the Dragons" and "The marriage of Jeyne Poole to Ramsey Bolton"
The Dance of the Dragons was a war between a Targaryen brother and sister who hated eachother and ultimately went to war with eachother. Dividing House Targaryen and almost ending its rule. If this theory is true there is a paralell with Dany, Jon, Tyrion, Jamie, Cersi and Faegon all reprising various roles of the origional conflict.
The 2nd is the marrige of Jeyne Poole to Ramsey Bolton under the pretense that she was really Arya Stark. Jeyne was origionally a servant girl and when rejecting Tywins proposed offer of Cersi for Rhaegar, Arys rejects it on the grounds that "a man does not marry his heir to his servants daughter." This draws a direct comparison as it shows that Aerys would view Joanna as a servant girl who had been legitimised by subterfuge (just like Jeyne Poole). Ramsey's brutal treatment of Jeyne is also reminiscent of Aerys brutal treatment of Fake Rahella (Joanna) which led to a Kingsguard soilder telling Jamie Lannister something along the lines ofthat "We protect her from everyone else, but not from him"
Other cues
Jamie and Cersi were origionally set to be married to Oberyn Martelland his sister. The Lannisters have little history with the Martells and after his wife's death Tywin rejects the previously organised match. Yet Rhaella wanting to marry her children into Dornish Royalty makes complete sense both due to Rhaella's personal relationship with the martell queen and that the Dornish were fierce Targaryen loyalists and would protect her children.
Jamie and Cersi Lannister often fit many of the cliche's used to describe Targaryens such as two sides of the same coin all the way down to their romantic relationship which was acceptable for Targaryens.
In book 5 Jamie supposedly talks to his mother via an obsidian candle. Obsidian Candles were magic and as we know magic use was typically reserved for those of kings blood. Typically that would refer to Starks and Targaryens but not Lannisters. So how were Jamie and his mother able to have that conversation without some element of royal Linneage?
Found a reddit post with a few useful quotes but they seem to not have pieced this part together. Not sure if i can post it.
GRRM has a wicked sense of humor and making Tyrion the one true king seems like his style.