Comments on this Game?

My opponent was banned for "fair play" violation, although I don't want to care about that - this game was very interesting, since I thought a decent human player was playing black, and I just want to care about that.

What strategic themes are important here? What moments were most critical, and how did I really misplay the position, in your opinion?

I think you had to go for 17.dxe4. Your Ng5/d4 idea leads to what I've seen called the "wedge" structure or "classical" structure. This should favor Black because he has full control of d5, chances for a kingside attack (even in the ending) starting with ..h6 and White has no obvious counterplay. Usually you would need to either jam a knight onto e4, try to undermine Black's control of d5 or be able to provoke queenside weaknesses but all of those things look hard to accomplish. As in the game 19.Bb5 Nce7 and 20...c6 repelled the threats.

Here's a good example of the dangers of the wedge structure:

Anyway it's a little scary to think that 18.d4 led to all this but it's probably the main problem.

I see many strategical mistakes. 9.d4!!should have played. After 9. --- e5 from black, d3 pawn become a backward pawn.  After move 8, your strategical plan should be attacking on 'c' file on his backward c7 pawn. ( your opponent may control center or attack your king side, forget it, if you can control "c " file better and lead to endgame , u can win) 21.Be2 is   another mistake, instead you should have played Bc4 and exchange that powerful kt. (dont worry about weakness in light squares after bishop exchange cos almost all your pawns are in different color and there is no danger for your king  with that black bishop.  At move 25 , your position is clearly in inferior position. So the strategy should be exchange the pieces or pawns as much as you can. Instead of Nd6, I will still try Nb2, and Bc4, and exchange that kt. After that Rc e1 and will play f4(if black take exf4 ep, you will have backward e3 but it can be protected by your king. After all, you already faded your initiative advantage by playing fianchetto ( it doesnt matter if you can play strategically in middle game but it matter cos you played above mentioned strategical mistakes and you almost lose the game) . I am  about 2000 rated player only , someone better than me can see/explain more in detail.