I’m gonna answer this backwards.
I don’t think he can transform into Femto, but I think Ganeshka saw him as Femto because he’d ascended into a higher plane of existence, in which he could see NeoGriffith for what he truly was.
So when we saw him as Femto during the scene where he killed Ganeshka and broke the world, it was because that’s what Ganeshka (and probs Skull Knight) saw. Like his true spiritual form, kinda thing.
Femto is a dick, yes, but tbf NeoGriffith has a ton of charisma. The way I see it, Femto didn’t need to be charismatic because he was a god on another playing field whose only peers were other completely dickish and shitty gods lol, like, when you only have Slan, Conrad, Ubik, and Void to hang out with, there’s no point in being charismatic. But as NeoGriffith he has a reason to be charming. I’m sure magic/fate/whatever also plays a part, but I also think part of it is just his natural Griffith-ness coming back to the fore. NeoGriffith is like what Griffith wished he was – perfectly charismatic, the absolute perfect image of a perfect king. Human Griffith did pretty well in that arena, so it stands to reason that NeoGriffith retained that charisma, and is making use of it again, with the added perfection that comes from being a god incarnate.
I never really thought about Femto being modeled after Guts. In the Black Swordsman arc he’s swole as fuck lol but I think that’s just the art before Miura really figured out the vibe he wanted. Everyone was huge in the Black Swordsman arc lol. During the Eclipse he’s slimmed down a bit, and then moreso in our glimpse of him with Ganeshka. I think it’s an interesting explanation though, and I dig it as a headcanon.
And as a contrast to his fragile helpless human body when he made the sacrifice it def makes sense for him to be reborn as a much more powerful, physically intimidating figure.