I’ve made a pass and made some slight adjustments. I did not change most of them, because there is a reason for some enemies to be easier (e.g. on “It’s Your Birthday”, the ribbon is made out of easier enemies to stand out.)
Yes
That makes no sense. If there’s no stereo panning, then sounds should sound exactly the same wherever on the screen they are.
Wing guns are either Gatling or Laser, and they always fire 3 times.
The pilot is a random enemy from the miniboss pool. If it’s a Gatling it fires 3 times, and if it’s a Laser 1 time.
Changed in v.156 
Idea
That’s not easy to answer. My 17-year-old GTX260 can handle this just fine, but I suspect there could be more recent integrated GPUs that have worse performance even though they’re much newer. I estimate that any discrete GPU made in the last 10 years should have no problem. But the only way to know is to actually test it on that particular CPU.
Also consider that on waves with many enemies and a lot of projectiles, collision checks might take up a significant amount of the time, so the frame rate much actually be CPU-bound rather than GPU-bound.
They disappear.
Bubbles vanish whenever there are still bubbles but no enemies remaining (the same rule applies to Barriers)
Bubbles disappear whenever the enemy/enemies they “protect” are all killed. They also disappear if they are on a periodic timer (e.g. Bubble Rings)
Note that a bubble can “protect” an enemy even if that enemy is visually outside the bubble. On “Bubble Bath”, the bubbles actually protect the ring of chickens outside them (as well as the enemy inside them)
Yes. Due to the wave’s construction, the bubbles must form straight lines when placed side-to-side rather than being placed perfectly centered on the enemy (centering would cause bubbles on Saucers to appear higher up than other bubbles). So they can’t perfectly inherit the enemy’s position, which can cause the enemy to move outside them.