While being in contact whit sea stone he is no more than a normal dude, so yes, he would have a so painful and suffering dead.

Seastone doesn't remove DF powers, it drains the user to the point of not allowing them to consciously control them. Luffy is stretchy when chained by seastone, but he can't stretch himself at will. Buggy can be chopped up when chained and be fine, but he can't levitate his bits or control them. It's the difference between being submerged in water and being touched by the user of the Yami Yami no Mi, which actually negates DF powers.

Seastone doesn't remove df powers it just makes them really weak. So no.

So no really buggy is permanently immune to sharp things no matter haki, seastone, etc.

I think he wouldn't be able to split

We just discussed this. His devil fruit powers would still be active and he gets chopped whether he likes it or not. Its not something he can really turn off if you know what I mean.

What if he got sliced by a sea prism sword?

Again, sea stone does nothing except make him tired. He will still split whether he wants to or not

I find people generally have more trouble grasping seastone until you contrast it to the Yami Yami's ability