We know that both players are getting a warning.
Brad is given a Game Rules Violation, for illegally targeting the Summit Prowler with Valorous Stance. He receives a warning. Likewise, Alice is given a warning for Failure to Maintain Game State, because she didn’t notice the error

On to the hard stuff…what about the fix?

This question split many people on their decision to back up or not. The decision of whether to back up falls to the Head Judge, and this is a situation where either solution is correct, when done for the right reasons.

The argument against backing up was presented very well by Andrew Keeler. Let’s revisit what he had to say:

From what I understand, backups should only happen if leaving things as-is is a substantially worse fix. This means that the burden of proof needs to be clearly in favor of backing up rather than not.
If we leave things as-is:
-Valorous stance ends up in GY
-Dromoka’s command ends up in GY
-Each player has no information about the other’s hand

Board is:
Brad- nothing

Alice-
4/3 Summit Prowler with 2 damage marked on it
2/2 Goblin
(Valorous stance will not kill summit prowler since prowler doesn’t have toughness 4 or greater, so it will be an illegal target on resolution)

Maintaining the purest boardstate always argues for a backup, and I think that most judges and players would instinctively want that sort of a result if at all possible (I know that I certainly do), but in this case leaving the gamestate as-is doesn’t seem to me to be clearly worse than backing up. Further, backing up will likely lead to a different line of play, since both players know the other has a card in hand that can interact with their card in hand (consider the indestructibility mode on valorous stance and the fight mode on Dromoka’s command).

Well said Andrew. What about the argument for backing up?

The correct reaction by Alice to Brad’s casting of Valorous Stance would be “you can’t do that”; instead, she allowed that error, and took another action instead. The players realized the mistake soon after that, so because it’s just one decision point, it’s pretty easy to rewind, and returns us to a non-compromised game.

And finally, the last question – if we opt to back up, how far do we go? The IPG can help with that.
IPG 1.4

To perform a backup, each individual action since the point of the error is reversed.

That means we put the Dromoka’s Command back in Alice’s hand, untap the mana spent to cast it and then do the same for Brad with Valorous Stance.