(5). Should thief be a class?
Probably not, but it's too late. The problem is trying to emphasize that the Thief class is not the thief profession; it's someone who is exceptionally subtle and clever.
The problem is the thief skills and the way they work. Or maybe it's the mental model people have of how tasks are learned and performed. If you learn how to do something, like cook a meal, you don't suddenly forget how. Learning to cook gives you the automatic ability to cook, and improving your skill means improving your speed, quality and efficiency, and your ability to deal with tricky ingredients or unexpected cooking situations.
We don't see skill rolls in pre-Greyhawk OD&D. What we see instead are rolls to see if something goes wrong, regardless of anyone's skill. Or, occasionally, rolls to change the otherwise inescapable, like listening at a door to get some warning about what might be lurking in an otherwise quiet room. These are rolls for things beyond anyone's power to control (without magic.)
What thief skills should do, in my opinion, is:
- Reduce the time it takes a thief to do some things anyone could do automatically (thieves should climb ropes faster.)
- Improve some things that aren't automatic, or make some of them automatic (Hear Noise improves.)
- Allow some actions no untrained person could do (pick locks, including "magical closures", as noted in Greyhawk.)