Sure these points makes a lot of sense, but there is one reason I am doubting Xebec being Im and thats the Outcome of the God Valley accident.
Sure Sengoku Said nothing about what happened to Xebec afterwards so he definetly survived the Gods Valley accident. But Xebec loosing followed by him senaking into Mary Geosis start to control the 5 Elders just sounds wrong for me. Thats why I prefer the idea of Xebec raising Teach after the war (it fits perfectly with name of Teach ship "Saber of Xebec"). So storywise it would also makes sense that Xebec is for Teach what Roger is for Luffy. So yeah I am not really a fan of the idea that an old geezer who already got defeated in the past becomes the final super villain
For me there are currently 3 potential endboss characters.
- Teach -> Pirate Endboss
- Sakazuki -> Marine Endboss
- Im -> Celestial Dragon Endboss
If you think of it that way then it will definitely look weird, but i didn't say Rocks sneaked into Mary Geose or anything
We still don't know exactly what happened in Gods Valley, it's possible the whole Island was destroyed on purpose to cover up Rocks survival
And that he was already going to sit on the Empty Throne
Another Villain similarity i didn't mention before is about their most loyal Servants, they are mostly not just Henchmen but Characters who share History with the Villain and want to fulfill his Revenge, Orochi's Kurozumi Elders & Kanjuro, Doflamingo's Executives, Enel's Birkan Priests, Arlong's friends from Childhood & Sun Pirate ... etc
Even Blackbeard, Oda made sure that all his Crew are Corrupt People who had a Job before (Corrupt Sheriff, Corrupt Doctor, Corrupt King, Corrupt Warden, Corrupt Bounty Hunter .... etc)
So the same thing applies for Im-Sama, he most probably had Allies from the very beginning, his most trusted Servants, none other than Gorosei! that's why i believe they were his Allies even before he sat on the Empty Throne
It's not like he acquired their Loyalty after, but they always wanted to make him King just like most other Villains
And i don't believe he is an Old Man, i also believe he had Ope Ope No Mi's Ability used on him, because let's be honest, there is no way Oda added such an Ability in the Story if it wasn't already used on someone and who makes sense?
Earlier theories claimed Gorosei were Immortal but after seeing them bow to Im-Sama, it's pretty clear he is the one who must have had the Operation done on him. This also explains why Doflamingo said that the Operation used on him combined with his Knowledge will make him rule everything, because he learned about Im-Sama
Finally, i don't disagree with Blackbeard being an End Boss at all, it's just that you're assuming Final Boss needs to only be one Character which isn't the case at all, Blackbeard will still be a Major Character in Final Arc and will be so influential same as Luffy
But if you re-check the Pattern i told you about, the Final Villain is someone who directly or indirectly killed Roger (Inspiring Death), a Revo Character is fighting against him and Shanks (Inspired Character who met Luffy) is related to him in some way
Just like Doflamingo --> Rocinante --> Law --> Kyros or Orochi --> Oden --> Momo --> Kin'emon or Hody --> Otohime --> Shirahoshi --> Fukaboshi .... etc
Blackbeard not only have his Pattern already with WB Pirates since he Killed WB (Inspiring Death) the Man who inspired Ace (Inspired Character who met Luffy) and Revo Character Marco who lost against him (Payback War) and all this resulted in BB replacing WB
And if we apply him to Overall Story, Dragon have nothing to do with BB, he aims to defeat WG/Im-Sama, this already show that Im is the Final Villain, not BB. Also Roger got executed by WG & fought Rocks in the past, Blackbeard have nothing to do with him or his Death.
You can say Shanks have history with BB, but Oda made sure he have history with Gorosei too, this is another Hint at how Im is the Villain that connects all Major Players. BB being inspired by Xebec is just Oda's way to make him related and add him as a Major Player in Final Arc too
Actually this could be the whole motivation behind Dragon creating Revo Army, most Revo Characters in previous Arcs fight because of the Knowledge they have that other Citizens don't, they know the Villain's Plans
So it makes sense that Roger learned about Im/Rocks & told Dragon not only about him but also CDs Origin which made him start Revos Army
As for Akainu, he is no Villain, please stop calling him one, seriously what did he do? is killing Ace something Villainous to do? The guy is a Marine, that's his Job, just because he have an Absolute Justice Philosophy doesn't make him a Villain, i mean look at Garp when Hina told him about Akainu's reply regarding Wano, he laughed and said that he expected such reply from Akainu, does this look like a Grandfather angry at Akainu for killing his Grandson? Ofc not, he is totally cool and no one ever said Akainu is Corrupt or unworthy of Leadership
Even if you say he killed Oharan escapees, he wasn't portrayed as Villain of that Arc or anything, it still doesn't change the fact that CP-9 & Marines too destroyed Ohara & all it's People, even Aokiji caught his friend Saul which led to his death (Assuming he didn't kill him himself)
I mean sure they will fight him and maybe he will resign or get removed and replaced by someone else like Fujitora maybe who is fit or Aokiji or whatever (Maybe that's why Aokiji was made younger than other Admirals) but it doesn't change the fact that Akainu is no Villain
Akainu's Role in Final Arc will be same as Fujitora's Role in Dressrosa Arc & Smoker in Alabasta Arc
Saying Akainu is a Villain is same as saying Fujitora was Main Villain of Dressrosa which isn't the case
Fujitora was helping Doflamingo just like Akainu will help WG while both hating them and eventually stop helping them just like Fujitora & Smoker stopped interfering Mid-War
Just like Alabasta Arc Quote
Those who lament the War (Civilians & Innocents & Kings/Queens)
Those who fight trying to remove corruption (Dragon & Sabo & all Revos Army)
Those who try to keep the Peace & Order & are caught in the middle of this War (Akainu & Fujitora & all Marines)
Those who are hiding in the Shadows causing this War & deceiving everyone (Im-Sama & Gorosei & all CDs)
Those who know the Truth, try to put it on themselves to stop this War & the Villain (Luffy & SHs & Allies who uncover the Truth)