Hey! Thanks for playing the game. I couldn't fall asleep and then I read your comment. Through the great jam organizers, I've pushed a hotfix that enables you to unlock the fourth mission, but you'll need to replay the third mission. Bright side is that you keep all of your chips from the previous time you beat it, so you can equip S0N1C with 3 speed boosts and just fly over that map once you beat it.
We're planning to keep polishing up the game (though hopefully no more hotfixes during the voting period) and the QoL issues you mentioned are very high priority, but I really didn't want anything breaking these past few hours... Actually, I'd already started working on different health bar colors.
Did you check out the upgrade screen? You can go there from the mission select, it's the bottom button. I would love to hear more of your thoughts on the gameplay!