@CalimTheCrystalGem
As told before, Steven's body is flesh, bones, blood, and not light, so if he is crucially damaged, he dies.
I doubt that Steven is completely made of human components, because otherwise, he would have never been able to shapeshift in the first place, let alone survive the experience.
He can fuse with Amethyst because he has a magical core of his being, aka Rose's gemstone. In fact, you can see that Smoky still has Steven's shirt, so when Steven fuses, his organic body isn't completly one with a light body.
It is unlikely that possession of Rose's gemstone would be enough to allow a fusion between a human and gem. Also, Steven's shirt not being to properly adapt to fusions is probably because the clothing is technically not a part of his body like those of gems.
His gemstone can problably be removed even when he is alive (it will leave him with a giant hole in his belly, though he will still survive) and, while he will loose his powers, Rose will be able to come back.
Your assumptions proved to be right on some extent, but still came out wrong on some parts; for starters, Steven's belly is not left with a giant hole when without his gem, but he still was left severely weakened by the lack of it, implying that their connection is much more complex than just a matter of placement. Second, while "Pink" was able to reform when separated from Steven, they came out a completely different person, even stating that their old self is now gone, which implies that the latter's birth somehow altered the entity inside the gem to some level.
Its basically Lapis' mirror all over again; Rose is trapped in a mirror (Steven), except that the mirror is sentient and can use her powers as he wants, like how Lapis' mirror spoke for her.
The analogy used here, honestly speaking, doee not make much sense, given that Lapis' mirror is not some mere object made magical, but a product of gem technology, therefore, the way it channels her energy is probably a lot more complex than we can imagine.
Edit: In addition, the fact Steven and "Pink" are able to fuse suggests even more that Steven is not completely human, and that the gem entity functions completely different from Rose.