3) it is also good idea, that you get the fist professional ability in level 3, not in level 1. So, you just cannot spend one level for some specific ability, you have to spend at least 3 levels.
I think this is an interesting idea. If expanded (see below), it could be used to replace the whole experience penalty concept.
Let me start out by saying that I do not like experience penalties. I have seen experience penalties for multiclassing in other games and I didn't like them. Let's forget about the specific interference with the problems with experience scaling in ADOM II at this moment. Let's say these are solved. Even then, experience penalties are still not a satisfying solution in my opinion. For players very good at powerlevelling, who know the ins and outs of the game and tricks (like wishing for an AKW or whatever), it will not be much of a penalty. At the same time, there is a risk it will lead to endless grinding for experience points.
So, how about this idea... Say that you started the game as a barbarian. Now you want to learn magic and you spend your next level-up on the wizard guild. Instead of directly becoming a level 1 wizard, you become a wizard apprentice (3). The (3) means that you need to gain 3 more apprentice levels before you become an actual wizard. You do not gain any wizard abilities yet. You still have a very hard time reading books, etc. You do gain hitpoints normally for your apprentice level, as well as skillpoints (based on the barbarian guild).
On your next level-up, if you choose wizard again, you become a wizard apprentice (2). If you're a wizard apprentice (1) and spend another level on wizard, you finally become a level 1 wizard. Now you become better at reading spellbooks, and gain any other advantages that come with being a wizard (skills, etc).
The number of apprentice levels that you need to "pay" can be different for every guild. This provides a simple way to vary the difficulty of multiclassing between guilds. You could say that the extra apprentice levels are still a sort of experience penalty, since you need to gain extra experience for those levels. However, the major difference is that, because of the level cap at level 50, the cost is permanent. If you want to join many guilds, you will spend many (most?) of those levels on apprenticeships and you will be left with few levels to gain actual abilities.