"My guess is the speed difference was probably too high for an accurate cannon shot."
They fire at, and hit, tanks on the ground, no?
"I think there's also a question of whether it would come down quickly *enough* with just some slow leaks in the envelope."
Then clearly you'll need more leaks. Rumo(u)r has it there are devices known as "computers" which could help you calculate how many such leaks it would take.
"Ideally you want to be able to bring it down for a quick, controlled landing"
Or just drop it just low enough to pluck it out of the air with one of your government of choice's heavy lifters. Line up four or five such aircraft in case the first one (two, three...) miss the snatch.
"so it doesn't drag across miles of power lines and such."
That's why you pluck it out of the air with one of your government of choice's heavy lifters. If it looks like it'll hit the ground anyway, time to give it a coup de grâce and try again with the next one.