I'm not sure I'd ever call a knight inferior though you're the pro. personally I've always preferred a knight's maneuverability in a crunch.

So good is 41...a5 that 40.c4 has to be labelled a blunder. However, it's not so easy to suggest an alternative here. For instance, consider this plausible line 40.Bd4 Bc1 41.b4 then 41...b5 will keep White's Q-side more or less permanently under restraint, giving Black a free hand on the other wing.

Actually I couldn't play 41...b5 because of 42.ab5, ab5; 43.Bd3 - but I fully agree that White should keep his Bishop pair. In that case the game is pretty much impossible to win (it was already extremely hard against only one Bishop).

I wanted to ask if 43. ...Kd8 44.c6 Ng4+ 45.Kf4 Nf6 would be playable.

Objectively yes - 46.Bf3, h6; 47.b5 looks drawish, but then again so does the game continuation. White can also play 44.Kf4 (with the threat of penetrating on the kingside), Nf7; 45.c6 (or 45.a5, which goes back into the game except for the position of the King), which means that this variation also needs to be calculated.

The move played prevents any kind of breakthrough by the White king. Such defensive moves have to be well thought out, though, as restricting one's opponent's chances sometimes has the effect of reducing one's own chances as well (I've had that happen!). However, Black's passed pawn couple ought to be mobile enough to secure Black a big advantage.