Will thieves be able to get a chance to hide in combat? Giving them a chance to backstab or at least hide one round, then attack the next, will make them seem less than "dead weight" in a party while still being able to contribute in a battle.

Also, will Intelligence and levels contribute to their ability to disarm traps? Having the ability to disarm traps only going up 1% per level when you also increase intelligence for them, learning more intricate ways to disable traps, would help to make them less a "dead weight" to everyone, when you can easily let a chest go, or cast a second level spell and call it good.

The original game didn't do either of these.

If you want the maximum chance of disarming a chest, you need a Thier (has to be a Thief, not a Ninja) with at least 16 Agility; that will give them a 95% chance of inspecting the chest trap, and disarming depends on level (with a +50 bonus for Thief/Ninja). So, 16 Agility makes the Thief as reliable as the CALFO spell.

In any case, most of the traps are manageable, particularly later when you have good spell access. Changing a level 11 Priest into a Fighter (to avoid Priest Blaster traps) will make the traps in the game manageable, so it's not a problem if the disarm fails, as long as you don't get teleported into rock (the worst thing that can happen in classic Wizardry).