This reminds me of this article by Jeremy Silman. The position you discuss is difficult to evaluate, as both parties have assets and drawbacks.
The first thing to notice is that the position is closed. Thus, if it doesn't open up, certainly black is not standing worse because of the two knights. However, these knights are severely restricted, and if you play:
[FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"]
1.e4 e5 2.Nf3 Nc6 3.Nc3 Nf6 4.Bb5 Bb4 5.O-O O-O 6.d3 d6 7.Bg5 Bxc3 8.bxc3 Qe7 9.Re1 Nd8 10.d4 Ne6 11.Bc1 c5 12.Bf1 Rd8 13.d5 Nf8 14.c4 Ng6!? 15. g3!
This last move deprives the knights of many good squares, and they do not seem so useful now. Moreover, we shall not forget that white already possesses the bishop pair, and any plans involving
b5 will open the position for the
Bf1 to come to life. Other plans for white to consider at this point are
Rab1 and at some point
f4 to blast the position open. Black needs to find active play to avoid getting excessively pressured by the bishops. This can go via
h6-g5, since all black pieces are on the kingside and it makes sense to start an attack there (plus, has the advantage of discouraging
f4 and keeping the position closed).
Now that we have outlined the main characteristics of the position, to your questions:
- Your analysis is correct as long as the position remains closed: black has more or less equalized. However, he cannot sit and wait, he needs active play to avoid getting too much pressure on the
b7 pawn and having the position uncontrolled when opened.
- The move
11. ... c5 is correct, but the pawn break
b5 is probably not the move you are looking for, since it opens files for both the
Bf1 and the
Rb1, that will inevitable end up there.
- The GM Akiba Rubinstein followed a very original play, by keeping a knight and the position closed, he benefited from the fact that the
c5 square is extremely weak, and a knight there is not easily dislodged. You can learn more about it on the above referenced article, but in general, black does not want to open the position.
- The position as it is is basically equal, black might have a slight advantage concerning the activity of the minor pieces (those two nice knights) but white has this same advantage about the major pieces, with the semi-open
bfile. It is up to the players to choose a good plan and implement it correctly, but white is easier to play from a practical point of view.