How does one win a coach's challenge?
Eddo: So Mike, let's say the ruling on the field is an incomplete pass; defense challenges, saying it was an INT; outcome is that QB's arm was NOT going forward, so it's a fumble. Do you "win" the challenge?
This gets pretty deep into the weeds of replay review. First, each part of the play must be reviewable; here, incomplete/complete is reviewable, and forward/backward pass (fumble) is also reviewable. The next part is what precisely is being challenged. In this case, the challenge is on a ruling of an incomplete. At this point, the coach making the challenge lets the referee know what he believes the result of the play should be; in this case, an interception.
Technically, the coach retains his time out if the ruling on the field is overturned in a way that has a competitive impact on the game. A re-spot of the ball with respect to the line to gain, for instance, is only successful if the result of the re-spot places the ball past the line to gain. Similarly, a coach challenging a double (or triple) fumble that was ruled otherwise will not win the challenge, even if the referee agrees the initial ruling was incorrect, unless the review results in a change of possession.
Your situation is interesting because the change in type of play (a forward pass to a fumble) changes the ability of the referee to rule on the result of the play. The standard for awarding a fumble recovery after review is clear recovery of the football during the play. So, if the "interception" was perfectly clear, then the coach would win the challenge, even though his basis for challenging was incorrect. However, if the recovery is less than clear, even if there is enough evidence to support an interception, the different rules for fumble recovery mean the coach loses the challenge and the offense is awarded the ball at the spot of the fumble.