Sorry to hear about bugs. It seems regardless of how much you test things yourself there will always be bugs.
I think I've triggered the boss bug when I made an engine update and have a fix for it. However, as I need to re-test everything it's unlikely I will release an updated version anytime soon.
Regarding control remapping, you are absolutely right, it should have been there. My engine has currently no support for it, but it's definitely something I should look into.
Finally I'm sorry to inform you that there are no extra levels or hidden features. The only thing that exists is unfinished levels or level concepts at my machine at home.
The game was actually made as a proof of concept for my game engine and after 4 levels I felt quite done with the game. There never was much thought put into the design, so the plasma gun is probably very OP. Foolishly I decided to make a shmup as I thought it would be easy. Boy was I wrong! A shmup requires constant action and variation.
As some time have passed since I finished Death Strike I must confess I have been toying with the idea of a sequel, Super Death Strike. Who knows, maybe it will be a spring project?