This is a really interesting platformer! I don't think I've ever seen the idea of enemies that turn into terrain when they die. I found it a little tough to control (I managed to get past level 4 using the trick you describe on the page), but it felt like I was wrestling with the slipperiness of the boxes now and then.
For what it's worth, I think box-based puzzle platformers (where you move crates around) will inevitably have some slipperiness and awkwardness in the controls. This might restrict play too much, but I think snapping the boxes to the buttons when they're close might be a possible solution to some of the issues where a small nudge would displace a box from a button (and close the door).
I think I cheesed a few levels by abusing the fact that there's a delay between when a switch is released and when the door actually closes.
There are some great ideas here, and I like the small mix of combat with puzzle! The art was also perfect, the animation of the bots transforming into boxes was great. Good work!