I fully understand where you're coming from. Believe me, when myself and my two friends used to run store level events, it was common for us to think, "Can we give a warning instead of a game loss" but you have to keep in mind, typically, you can't really do that at a higher level event. Regardless, I think you're still missing the point here.
First off, players shouldn't be asking you to give a game loss. Rule sharking tends to be looked down on, and depending on the situation, could be grounds for a penalty, but that's a whole different discussion. The whole point is. It doesn't matter who could win the match. The game stops when you're called over. Anything that could happen, be it winning or losing, does not matter. It has no relevance to determining the ruling. In this situation, you'd do the following.
1. Get statements from both players.
2. Rewind the game to the point where the extra card was drawn.
This means literally reversing the game play. Once you get to the point where the extra card is drawn. That's it, the game's time line is stopped. At this point, the fact that an ability was played by A has no relevance, because at this point, it's a future moves. Future moves have no relevance to current game state and ruling.
3. Like I said, attempt to determine the extra can, if you can, it goes back, he gets a warning, the game goes on.
If that means A playing his ability and winning, then that's that. It's out of your hands, it's a future move. The game was reset to a point where it would be the equivelent of A never drawing the extra card. Your ruling does not affect the outcome in this case. Yeah, maybe B us upset, however, your job is to explain the policy regarding drawing extra cards to him, and explain you were following established guidelines. After that, it's done it's over, there's nothing more you can do about it.
If it means giving A a game loss because the card can't be determined, then in this case, being a match, the players are 1-1, they both could still potentially win. A learned his lesson, B is happy that you're following established guidelines. The situation is done. Either way, the tournament and life go on.
Either way, the point is still in place. You can not let future moves determine your rulings.
Go Player ~ 22 Kyu