Problem 1 seems way too circumstantial to matter.
1) Your character must have low recovery and you are utilizing it
2) The barrel must be in that exact position
3) It must be facing upside down
4) The camera must be hiding it, else you would know (and if you're recovering low, the camera would normally follow you)
Yeah, if it's really needed, you can just play in fixed camera mode. But... that's not going to ever really be needed
.
So, problem 2...
Initially, I thought being out of a stage strike for these character would stink. However, on closer inspection, these 4 characters do not benefit insanely from other specific stages. You don't need to ban Delfino or Halberd, because the characters do not benefit from the low ceilings.
Against Jigglypuff, you'll want to ban KJ64 and Battlefield. All the others are relatively neutral.
Source. Despite what you may think about Rest, Jigglypuff actually does not like low ceilings.
Peach you probably want to ban the same stages as Jiggs, and you'll be fine.
Source
Villager is a bit different with this.
Source. He likes Battlefield/Lylat Cruise a lot. At the start of the thread, there was some people saying KJ64 was actually bad for Villager, but later some saying it's decent for him. Stalling with the cannon was never brought up in the thread, though.
Kirby does not have much information on stages at all. They have a long MU/Stage discussion thread, but in the past, when I went through it all, I only really picked up that Battlefield seems good and T&C
might be bad. Then, the only other information is either about 3ds stages or that
apparently, Kirby can get some early U-throw kills off KJ64, T&C, and Duck Hunt? I'm not sure if that is something Kirby really looks for or something. Well, I guess you may have wanted to ban KJ64 already to begin with.
Coincidentally, KJ64 + Battlefield seem like the best choices for striking with all 4 characters. Of course, this depends on what character you yourself use. Interestingly, the only character mains that appear to be aware of the cannon stalling are Jigglypuff mains, right now.
The only character of these 4 that banning KJ64
might be a bad idea is Villager. If they do not know about the cannon stalling or consider the stage to be bad for them, striking the stage might be a waste when you could have striked either Battlefield or Lylat instead. If your tournament allows 3 strikes during counterpicking, then it is no decision at all. Otherwise, you might have to either let Villager play on a stage he likes or risk the small chance that he would know about cannon stalling, would actually do it, and would have the circumstances necessary to actually use it.
From what I have found today, my worries over having this stage legal have completely vanished. There is no reason to have this stage banned whatsoever. In practice with the striking system, cannon stalling presents no issue for the players at all.
Note:
I would like to note that a character will simply pass through the cannon if another is already in it. If a character commits too largely to the cannon stalling, the character might not be able to make it back to the stage without using the cannon. In that case, you could kill the person by just going in the cannon yourself.