An excellent concept document covering all the topics and information required, and gives an excellent description of how the game will work, well done.
The game idea sounds like a fun take on the genre, although I'm sure this setting already exists in bullet hells somewhere.
Definitely a very feasible concept, and the addition of the choice of powerups will be great. My recommendation is to not overcomplicate the way those powerups are coded, given the scope of the game and the time available.