The reason why I suggested Actionscript is because it is very very easy to pick up (I developed a scrolling shooter in flash in < 3 starting with 0 programming experience), and there are lots of tutorials from which you can just cut and paste codes: http://www.gotoandpl...shooter_p02.php. And you can always go to thriving communities like Newgrounds to ask if you get stuck.
In particular, by cutting and pasting the code from the tutorial here: http://www.gotoandpl...shooter_p02.php, you will be able to set up the basic shooter pretty fast. After that, I can see how you easily prototype the other features:
1) For a start, fix a list of songs. Randomize the list at the start for variety.
2) Pre-program the tempo by hand for the songs on your list in advance for simplicity.
3) Off the top of my head, I might code the game by having a "tempo" variable. And make the variables that control firing speed, damage etc dependent on this "tempo" variable.
4) Again, off the top of my head, the toughest challenge might be the territory membrane. Not sure how you would implement it in Flash. But from past experience programming shooters in Flash, you should be able to create oddly shaped hitboxes VERY easily. If you get stuck, post on Newgrounds forum and get replies/help in literally minutes.
Now if you polish the game really well (lots of artists and musicians to help you for free on Newgrounds), upload it and it gets really popular, there are funding for a sequel that you can apply for. Or you can use it as a springboard to make the full game.
Good luck! I would really love to a prototype of this in action. Message me if you need testers.
