Hiding makes sense if the Thief actually tries to hide from an opponent's view (not necessarily hiding in a dark spot, maybe moving to some blind spot or mixing with the combatants), it will take him his action and the result wouldn't be known to him.
If he just advance and attack a distracted opponent then a backstab is ok to me.
If he then just sit there, or moves around with no other precaution, then no, his opponent is aware and can't be backstabed.
Surprise is rolled at the start of an encounter unless you want to roll it for every "sudden" situation.So I've been thinking of maybe just rolling a d6 every time for surprise but allowing the thief to achieve surprise on a 1-4.
Some games gives sneaky characters like Elves and Rangers a 1-2 chance on the d6, that would be ok for a thief, but the party would have to move carefully or at a distance behind the thief.
If you don't want the thief to backsab more than once per opponent then simply don't allow, consider that a backstabbed opponent will remain alert enough to cancel this special ability instead of dealing with rules alteration or additions.This makes more sense to me in the heat of combat and also not allowing the thief to backstab the same target more than once. What do you think?