Thank you for playing and for this review! While all hearts are optional (and yes, max is 9, because otherwise I would have to make the health indicator 8 pixels wider and I did not want it to obstruct the view any further), upgrades are unavoidable, i.e. it is impossible to complete the game without them.
Regarding spikes bug, it is only the spiky top part that hurts. It probably makes sense, but it is not something one would expect from spikes in a game, sort of like the "reality is unrealistic" trope. I was thinking about expanding their collision box to full 6 pixels of height or explore it as a mechanic, but ended up doing neither, partially because I forgot about that.
Once again, thank you for the comprehensive review! :)