Ehhh, technically but its hard for them to tell if your able to switch back before turning it in. And if it doesn't boot then well... It's not going to be much of an issue then. Also it is a bit legally grey if companies can void you warrenty solely for installing a custom ROM.

Depends on the brand. Xiaomi will (in most cases) service the phone even if the bootloader was unlocked and os changed, you just need to restore it to stock state. I haven't personally tried this, but I heard multiple stories confirming this.

It shouldn't but, ya know capitalism