If they are going to violate parley anyway, why not some more foolamancy and then a yellow-dragon dive bomber vs. the tower - with the King and the Prince standing right there in plain sight?
Because they cannot attack the tower. The zone between the airspace and the tower would stop all attacks and any acidic battle crap would slide and fall onto the ground below. The only reason the battle crap can hit the garrison is because it is FALLING. Their is no falling exploit between the Airspace and Tower (at least non mentioned) only between the Airspace and the Garrison.
There IS a danger, and they CAN attack the tower using the same mechanic. If there was no danger, Jetstone would not have ordered the yellows away. Trammenis outright states that the Yellows could hit them on the tower.
I'm not saying it's as good as Parson's plan, but it seems a lot simpler and less risky. Depending on how much juice Jack had left.
Assuming that they can attack from airspace to tower using battlecrap (which I still maintain they cannot - Trammenis was just being overly cautious) they would not be able to select targets and could not guarantee taking out Slately. It would also probably mean an immediate reprisal from the tower defenses, the archers (aided by the Dittomancer) and the Healomancer keeping units alive, resulting in the end of GK's units including Wanda and Jack.
Edit: Yes, the yellows can probably bomb the tower from above but with no guarantee they would hit Slately and would most likely result in immediate retaliation via the Tower defenses and possibly the archers.