Glad it could be of use.
I did not download the game, unfortunately. Maybe I'll give it a try later.
Immunity would be a good move. My game has it, and it allows for a much fairer experience.. although the problem could be that it could nullify the use for multiple projectiles - which I think is a good effect already. Maybe some of them can just be there for decoration - some of them can damage the player. This does come at the cost of slight readability issue - which would be another issue you'd have to solve.
I didn't realize there were multiple bosses since I couldn't get past the first boss. Variable jump height can work, but I think the minimum threshold could probably be brought up a bit.. the lowest point might as well still be grounded.
I understand that it is a little play test, so lack of sounds in some areas could be forgiving - however, I think audio design should be part of the play test at some point.
A video game, I think, should be treated much more like a cohesive experience than a set of mechanics working together.
Personally, I wouldn't release a public build until I had something more representative of the full experience. I'd leave more mechanical testing to private builds you share with friends or family, although, I do understand that mechanics is something you'll still be constantly working out even after you go into public beta.
You don't need all of the polish there, but I think there is a point where the player can get the gist of what the full experience should be - a boss battle, for instance, is certainly mechanics - but it is also much more intensive emotionally compared to other enemies in the game. If you're game is a boss rush, then you'll have to figure out how to make each boss distinctive, not just visually or mechanically, but emotionally as well - I'd assume that each boss you encounter should be bigger and more intense than the previous one.
The real question I think you should be looking to answer in a public beta "is this game delivering the intended experience or not?" Outside of bug fixes or working out general clunky-ness, I think you'll be able to come away with a much clearer direction of where to go with your project. This is just my take away on the matter and how I'd go about it, though.