1. Is it difficult for you to pronounce this letter?
No.
2. On Youtube I have heard some singers pronounce this letter really loud, ...
I believe that they just articulate too much when on scene, i.e. they try to pronounce everything as clearly as possible. There's also so called scenic pronunciation, which is taught to TV hosts, artists, etc. - basically it's a very good and even exaggerated diction (in real life people mumble, slur their vowels etc., which does not sound good on screen).
A folklore [...] says that even Russians have to learn to pronounce this letter, and that a small number of Russians, including Lenin, never learned it.
Russians have to learn to pronounce ALL letters when they learn to speak, and some kids have problems with some sounds. But it's true that some people never learn to roll their R's. It's considered a defect of speech (along with lisping and stuttering). But it does not stand out too much or affect understanding in any way, and many people even think that it sounds cute, so I would not worry about it too much.