I have exactly the same issue as the person at The volume button ist stuck whose issue was closed, despite there being no answer that actually said how to resolve it.
My volume button is stuck and if I try to start the phone it always enters into the reboot menu, I guess because the button is registering as pressed all the time. I tried to disassemble the upper part of the back, but I cannot reach the buttons.
Has anyone resolved this issue previously? Is there any way of accessing or manipulating the buttons so that I can work out what’s wrong, and readjust it?