One more question. Is it OK to reference a particular user in the text of a question (right at the end of the post with an @), just to be sure that that user gets to read my question (he receives an inbox message, I believe) and increase the chances to extract a thorough answer from him/her? It is not necessarily unfair to the other users, I would still choose to upvote all answers and choose the most satisfactory one.
This is not a feature of Stack Exchange, and I don't really think it should be.
This is covered well in this answer/question on the main Meta.
This isn't really a one on one tutoring site, it's all community driven. If that user is active enough, they will likely see the question anyways if it is one they would usually answer. They may have contact info in their profile if you really want their opinion (but they may also not want to be contacted for that purpose, so respect that if mentioned). You can also check the Chat to see if they are online there and willing to take a look.
You can't really prompt a specific person to answer your questions unless they already make themselves available to do so. That seem fair to me.