2)Could it be that in a good teams (I'm in a very weak league, right now I won all of my 22 games, ended in semifinal in a cup and will play CL final soon) goalkeeper just meant to have low ratings? Maybe average rating of 6.5 is not so bad?
You 're right. I also have a good team (of 5*-6*), I win my leagues and accepting very few goals every season.
I have only one GK and he usually gets rates of 6-7. Sometimes an 8 and rarely a 5.
A GK in a good team with good defenders is more "useless" so the game doesn't give a rate because he keep his posts untouched but how active and effective he is.
If he starts getting 4 & 5s then it's dangerous to loose any future game so must bench asap.