[Radek]

AFAIK, there is no difference among "trainers". All skills work on the basis "successful action - a predefined amount of skill given". Therefore, if you block a golden saint then you get the same amount of blocking experience as if you block a rat. My knowledge is based on tables published on the internet and it may be wrong. Nevertheless, I haven't seen a difference in gaining a skill experience between a golden saint and a rat. The only difference I have experienced is based on the skill position. You proceed in maior skill fastest and in miscellaeous skill slowest.
As far as the level adjustment is concerned, I have seen only adjustments caused by the skill level, not by the character level. Unlike Daggerfall, a high level character seems to proceed in skills as fast as low a level character. Again, I can be wrong.
As far as your example with alchemy is concerned, it's the matter of the game logic. If the game logic is trivial and allows increasing alchemy by killing dragons then you are right. If the logic is less trivial, then you will have to sacrifice your skill points for Alchemy even if you need them somewhwere else. If the game logic is even less trivial and allows self training, then you get a picture, which makes perfect sense.
Let us note that, for example, Might and Magic system is more fair than the Morrowind one. In Might and Magic, you have to act if you want to level up and get skill points. Only then you can distribute your skill points among skills. In Morrowind, you can sit in Balmora, earn money by selling and buying items at a bribed shopkeep and use the money for training skills to 50 or more. It can happen easily, that you will leave Balmora for the first time at level 10 or so. It doesn't make perfect sense.