The first thing that has to be said is that your "How to play" section, unfortunately, did not teach me how to play. I didn't realize it was possible to shoot until my second time playing. That said, I had more fun with it before I knew how to shoot. The natural risk/reward of waiting around to mine a resource or dragging your partner to safety was great. The only problem was that after a while, there were a TON of enemies, and at a certain point it was impossible to move because they just covered every occupiable space on the level.
Once I learned how to shoot, then I started noticing things like not being able to hit my targets because of weird level geometry, and my buddy getting stuck on corners and clipping out, and not being able to mine from anywhere but directly above the crystal. This might be a case of less-is-more. If you reduced this to one to three enemies, and took out the shooting, you could generate some serious tension, and a real sense of having to herd your big liability brother around.
(Small thing: I wish this had been uploaded to itch.io instead of google drive.)