I wish I could answer your questions with clear evaluations but then I would be Leko at least

. Despite I had your diagram positions in several games I feel like a woodpusher and know deep insight how complicated chess is.
OK - I like to give some suggestions to keep this thread rolling:
Quote:
Q1:
13...Bb7 14.d5 Rac8 15.Bd3 Nd7 16.Nf1 f5 17.exf5 Bxd5 18.Ng3
What is the evaluation of the following position and are there better alternatives?
The better player wins is the only thing which comes to my mind here. I somehow remember that playing for ...f5 in this variation (I think it's called "Panov") is somewhat dubious but I cant remember why - perhaps someone else does...
Quote:Q2:
13...Rd8. Should white sac the pawn with 14.Nf1 exd4 15.Ng3 Nc6 16.Bb3 Qb6 (16...Be6 17.Bf4 Qb6 and black cannot take on b3 because he loses control over f5) 17.Bf4 Be6 18.Bxe6 fxe6 19.Qb3 d5 20.exd5 Na5 21.Qd3 Nxd5 22.Bg5 or play 14.b3 instead?
I think both moves (14.Nf1 and 14.b3) are possible (I think one is recommended in Khalifman's book). I always play 14.d5 in reply to Rd8 to avoid complications.
Quote:Q3:
What is the concrete reason for playing 14...a5 in this mainline chigorin position? It looks as if it weakens the b5 pawn.
Yes b5 will be weak and there are classical examples where White exploits this (there is a grandious Leko-game some years ago). On the other hand Black starts counterplay on the queenside and conquers space there - again as with "Q1" the better player wins.
Quote:Q4:
What is the strategy for both sides in the following main line position?
As always in the Closed Spanish White has the possibility to play on two flanks. Furthermore he can act in the centre when he likes. Black needs to carefully defend his position and observe White's piece movements (and keep an eye on b5). If the time comes he may release the tension in the centre with ed4 followed by d5. And I like to continue my monologue: The better player wins.
Closed Spanish - Wonderful to play with White and Black!