(KW, June 15th)
OK, but what if 13. Nbd2 Nc3 14. Bxc3
(pretty well forced, exchanging our Bishop trained on the Kingside for a Knight, otherwise Black has succeeded to get another piece into play on the Queenside, and on an Outpost to boot!) 14. ... bxc3
and now our Nd2 is forced to move ...
Following natural moves, if 15. Nc4 Bxc4
(...dxc4 could be triple pawns!) 16. bxc4 Qa5
pinning the capture on c3, and both sides have doubled pawns on the c-file! Black could follow later with Nb4 (Outpost) and Rb8 (Preparing to double Rooks on Open File). I do not like this position (seen Diagram 3-4a, below) ...
Black could follow up with Nb4 and Rb8.
The tension of the resulting c- and d-file pawns in the center would have to be resolved, and White could force a Backward d-Pawn with say 17. cxd5 exd5 dxc5 but upon Qxc5 (see Diagram 3-4b, below), Black controls every Center square (even d4 upon d5-d4), not our objective. The game would not be under our control at all.
After 17. cxd5 exd5 18. dxc5 Qxc5
Black controls every center square.
I checked Nfd2, but since that takes a defender from d4, every variation had problems (I won't go into them here, but will let you work through them).
So what about 13. c4 bxc3 e.p., 14. Nxc3 Nxc3 15. Qxc3
? Would that work? The following (Diagram 3-4c, below) will show you how I think through tactical positions ...
After 13. c4 bxc3 e.p., 14. Nxc3 Nxc3 15. Qxc3
White is now threatening 16. dxc5
with a mating threat on g7. So Black could go 15...cxd4 16. Qxc6
winning the Knight, and after exchanging Queens, White would have a Rook on the 6th rank causing havoc. Notice how both Black Bishops have few targets (our earlier dark square strategy is now kicking in).
The other option for Black is 15...c4
(Diagram 3-4d, below) blocking the d4-pawn from advancing.
After 13. c4 bxc3 14. Nxc3
What are Black's other moves?
Are we missing something before Move 15? Let's see.
If 13. c4 Nc3 14. Nxc3 bxc3 15. Qxc3
we are in a similar position, with the further advantage of the pawn on c4. Black will probably not move this way.
The c4-pawn is attacked twice and defended twice. The d4-pawn is attacked 3x and defended 3x. Won't do either of those, so 13...bxc3
is what Black will move.
So 14. Nxc3
, what are Black's other moves other than Nxc3?
The pawn on d4 is attacked 3 times and defended twice (currently, with the Knight in the way - when you calculate variations, consider what has moved, and what has changed in protection of a-pawn, piece and square).
So Black could move 14...cxd4 15. Nxe4 dxe4 16. Nxd4 Nxd4 17. Bxd4
(Diagram 3-4e, below) and the Queen will have to move, White threatens to take the e4-pawn, and controls the c-file for now. Not a pretty position for Black.
After 14... cxd4 15. Nxe4 dxe4
16.Nxd4 Nxd4 17. Bxd4
Probably best here is 15. Nxe4 Qxe1+ 16. Nxe1! dxe4 17. Bxe4
(Diagram 3-4f, below) and White has a commanding position, better pawn structure, better Control of the Center, and his Bishops are lined up against Black's Kingside.
15. Nxe4 Qxe1+ 16. Nxe1! dxe4 17. Bxe4
Based on all of the above, I am recommending c4.
This is an extensive analysis, but not exhaustive (but it IS exhausting!). However, it looks like we have good chances in what has been calculated.