Thanks for the compliment on the music and art style!
As for the boss, If you click levers in the boss room you regenerate health during the attack.
To nullify homing missiles - run them into the boss or other enemies or reflect them into the wall.
And as for the attacks - I originally had them switched, but the slow attack I found to be more useful against flying enemies and OHKO enemies (like the beetle and butterfly later on). Higher range could be a good idea though. Or maybe just keybinds so you can control the attacks.