The only way I see to stop the plan described by Estragon would be 41. ... Qe7 42. h4 g5, followed by Kg7 to free the queen. We will lose the a-pawn after Qa4, but it is not clear we are lost. This does seem the most difficult line to deal with, and hence the most probable. I don't see what he described as a zugwang, as g5 would win by force even if we could pass.
40. ... Be7 does indeed look a playable alternative to my suggested divergence from your earlier line, dsarkar. We need to find which line is the more secure draw, in the event that we approach this variation.
[after 36...Rcxb6]
37.Bxa6 d5! 38.exd5 e4!
39.Re3 Bd4!
40.cxd4 Rxb4
40.Rxb6 Rxb6 41.Qc4 Qxc4 42.Bxc4 Bxe3 43.fxe3 Rxb2
39.Rf5 Qd6!
40.Rxb6 Rxb6
40.Bb5 (Elroch's line) Be7!
39.Rxf6!? (may be best reply to 37...d5 line) 39...gxf6 (39...Rxb4 not good)
40.Rxb6 Rxb6 41.Qc4 Qxc4 42.Bxc4 Rxb2
40.Qc4 Qxc4 41.Bxc4 Rxb4 42.cxb4 Rxb4 43.b3
40.Bc4 Rxb4 41.cxb4 e3!! (42.fxe3 Qg3+)
40.Be2 e3! 41.Rxb6 Rxb6 42.Qc2 exf2+ 43.Kg2 Qb7!
40.Bf1 e3!
41.fxe3 Qg3+ draw
41.Qc4 exf2+ 42.Kg2 Qe5 (threatening f5) (42.Kxf2 Qh2+)
41.Kg2 exf2 42.Qc4 Qe5
40.Kg2 (white's best) f5 (41.gxf5 Rxb4 42.cxb4 Kh7! 43.Be2 Rg8+)
37.Rxb6 Rxb6
38.Qa2
38...Qb7 39.b4 Kf8 40.Bd5 Qd7
38...Kf8?! 39.h4 Rxb2 (forced) 40.Qxb2 Qxc4
38...a5? 39.Qxa5
38.Qa4 Qc6!
38.Bxf7+? Kf8! (Kh7/Kh8 loses) (39.Q-move Qxf7)
37.h4
37...Rxb4 38.cxb4 Rf8!
39.Bxa6 Bxh4 40.b5 Qd7
41.b6 Qxg4+ 42.Kf1 Bd8 43.b7 Bc7
39.h5 Qc6
37...a5?! 38.Rxb6 Rxb6 39.Qa2 not good
37.Bd5 Rxb4 38.cxb4
38...a5?! 39.b5! Qa7 40.Qc4 Qd7
41.Rb3 Bd8 42.Kg2 Qc7
41.Kg2! (41...Qxb4 42.Bxf7+ not good)
38...Kh7!? 39.Bxf7 Qb7! 40.Be6!
40...Qxb4? 41.Bf5+ Kh8 42.Qf7
40...Kh8 41.Bf5 a5!? 42.Qe6 axb4
43.h4 Qe7
43.Rd3 Qe7 44.Rxd6 Qxe6 45.Rxe6
38...Kh8?! 39.Bxf7 Qb7 40.Bg6! Qxb4 41.Qf7!
38...Rf8?! 39.Rc3 Qa7 40.Rc6
38...Qa7 39.h4 Rf8
38...Qd7 (prevents h4)
37.other?! (just for records) Rxb4 38.cxb4 Qb7
37.Bd5 line is not yet complete, and might require further tweaking