I can picture a rules lawyer that doesn't want the opening in the second scenario, and so does the final damage to make the opening at the end of the first scenario, thus denying the attacking player the ability to say they caused the opening.
The scenario does not specify that damage that didn't break the wall down carries forward, so an obnoxious rules lawyer could insist that a defender doing even a single final point of damage to the wall would block the opening for the next scenario, because the attacker did not cause the actual opening.
Rules need common sense, or outside control, or this kind of idiocy can take a lot of fun out of gaming.
I keep it simple for competitive leagues, because I stand as the final rules arbiter, and I would carry any opening forward, regardless of who caused how much damage, because the opening had been made. I'd also give victory points to the opposing side if someone shot at and destroyed a unit from the same side for any reason with the same kind of common sense ratianale; it doesn't matter who did it, it still gives the same advantage. Friendly fire isn't, but the opposition always gains and is happy to let you do it.
The only situation that would leave exploitable would be friends playing against each other in the first game, and the defending player opening holes to help their friend in their next game against a different player. It isn't such an advantage as to break the game, nor would it be easy to verify as fully intended as cheating, but it would certainly flag the defending player as someone to watch for further unsportsmanlike behavior that could get them banned from future events I ran.
For (questionably) friendly games against a player like this, before sides are chosen, make sure the impact is discussed before the game begins. If they seriously play this kind of idiocy, I'd suggest finding new people to play against for friendly gaming.