The rules say "no pressures" this is to be a fun experience for the times we really feel the need to make a bit more and make experimentations. Having that said I too am a little too much absent, sorry! December was a tough month here.
Regarding your questions:
We need waves
Enemies should come in "waves", let's say 3 waves of action close together and 1 wave of pause for resting.
A game easily gets very tiresome if we need to be focus and active all the time.
Between each wave we can have some coins to catch and/or some obstacles.
Our hero ship can rotate 90º easily for each side, so we can make some really tight areas for it to navigate that requires to use that rotation feature ( I can even think in a spiral zone that it needs to rotate full 360º to pass trough it :). Coins can be placed to guide the player.
If it touches the obstacle the ship looses shields energy until eventually explode.
You can stage it with cubes, spheres, circles, cylinders etc that I'll improve the objects later.
It doesn't need to be realistic in any way, Super Mario platforms just float in the air by pure magic and that's how it should be
Obstacles stay at the same level than the ships and we can keep improving them and background scenery separately.
Close of the end of the level we can have a bit of enemies and obstacles at the same time.
You are correct, player and enemies should be smaller.
Burst fire can be used as an obstacle. For example a slower enemy ship could fire a burst of parallel lasers kind of drawing a road.
I'll verify your version right after pressing the reply button
Updated the main file with a scenery correction.
I've noted that one of the bridges were in the way of the ships, so I've adjusted it:
Apart from the bugs (that prevent it to play for long periods of time and now seems to disables shooting and explosions), everything looks great