How would you annotate this move?

1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 d6 5. d4 exd4 6. Nxd4 Nf6 7. Nxc6 Qd7

6...Nf6 is a blunder, but 7...Qd7 is actually a good move considering the options. So do you just label it "Qd7!"? Is there a way to say "best move by losing side but what is the follow up?" Something like, "Qd7& then?"

It's not obvious how white should respond.

There should only be 2 annotations: mistake (?) and blunder (??).
5 d4? is a mistake, loses a pawn.
6...Nf6? is a mistake: misses a win

'I have attached question marks to the moves which change a winning position into a drawn game, or a drawn position into a losing one, according to my judgment; a move which changes a winning game into a losing one deserves two question marks ... I have distributed question marks in brackets to moves which are obviously inaccurate and significantly increase the difficulty of the player's task ... There are no exclamation marks, as they serve no useful purpose. The best move should be mentioned in the analysis in any case; an exclamation mark can only serve to indicate the personal excitement of the commentator.' - GM Robert Hübner

I have no problem with 5 being a mistake. Looking past that though, I see 6...Nf6 as a clear blunder and not a mistake. I like to keep these 3 categories of inaccuracy, mistake, and blunder as starting points. Obviously, a blunder of a rook up 10 is different than a blunder of a rook where the position was 0.00 before. In this case, we have a blunder of the knight. Maybe this is splitting hairs, but it's like saying "several" vs. "a lot". The implications are different.

Accepting those three categories, my question centered on 7...Qd7, not moves 5 and 6. How would you annotate 7...Qd7 where the follow up is not obvious?

@3

"I see 6...Nf6 as a clear blunder and not a mistake."
++ Mistake (?): turns a drawn position into a lost position,
blunder (??): turns a won position into a lost position.

"3 categories of inaccuracy, mistake, and blunder"
++ Inaccuracies do not exist. Either a move changes the game state draw/lost/won, or not.
If it changes the games state, then it is a mistake (?), not an inaccuracy.
If it does not change the game state, then it is not inaccurate.

I disagree that a blunder ONLY turns a won position into a lost position.

Can Magnus win against Stockfish with queen odds in favor of the engine? We can debate if it is a handicap or blunder in that case. Just put the queen loss at say move 25, and I would say that is good enough to call it a blunder instead of a handicap.

I can see where you are going with the inaccuracy comment, but it helps to categorize human play from best play (engine/theory). We could look further into this by looking at chess puzzles. Some can have mates in 7, some in 5, and I lost one where I had a mate in 17. In that case, I was inaccurate with my strategy, not the moves. The moves, according to your definition, turned it from a won position to a lost position. Yet, both labels are valid for different reasons.