You raise an interesting point, there should either be no max health at all, or the heal option should not show up unless you're down 3. I'll also be adding a reroll, I think a coin is a high cost but I guess it's still better than missing an entire upgrade. Maybe I'll increase coin availability to compensate.
There should not be a limit to the number of multi shot boosts you can take, I think you've just been unlucky. Maybe what I'll do though is have each subsequent boost give you more arrows, so multi-shot initially gives you 2, then the upgrade gives you +2=4, then the next is +3=7, and so on. Combined with +damage upgrades, that could really give you some crowd clearing ability.
I may experiment with mixing upgrades (such as acid spreading fire), I'll need to think on that one.