is it really farfetched that memory manipulation could cause someone to adopt a different voice?

I'm not saying Frank couldn't suddenly have another voice, in fact I'd imagine he would have a different voice if he appears in TPP, it's just the fact that it would be THE same voice as Big Boss. It would be silly for two major characters to have the same voice. That would be like having David Hayter voice Solid Snake and Big Boss in MGS4 or even a Metal Gear/MG2 remake. You could argue the medic was also Kiefer but he was only a minor character and at least had a pitch change. '

if BB was acting different I'm pretty sure Ocelot would just go along with it and not question it

I'm saying that if past Metal Gear's are any indication, Ocelot would be part of a big twist like that. C'mon, it's Ocelot we're talking about here.

it isn't apparent that Big Boss (at least from the MGS1 art etc.) has a cybernetic arm

Well actually, as far as I'm aware Big Boss (even in the MGS1 concept art) only appears with a trench coat and gloves so there's not exactly a way of telling. And even if he was shown to have a real arm, Gray Fox isn't shown to have a cybernetic arm or a missing eye in Metal Gear 1 and 2, so.....

And as for why I don't want it? Well The Phantom Pain is supposed to be about BIG BOSS' path for revenge and his transition to the villain (or probably more accurately, 'anti-hero') we know from Metal Gear and Metal Gear 2: Solid Snake. I disapprove of the theory because like I said the "evidence" can pretty easily be shot down.

Also, about the Gray Fox bust being in Paris, did you ever think that he received it while over there? The bust didn't even appear in Kojima Station until episode 4. And how does Kojima taking photos of the TPP PS4 and arm replicas with the bust in the background prove anything? Is he not allowed to take photos in front of the Kojima Station couch where the bust is located?