Really like the idea, it feels pretty unique. I did have some trouble wrapping my head around was going on at first -- at the beginning (three lever puzzle), I was getting stuck in the wall and that was adding to the difficulty of hitting the switches, and then later when I was spamming click trying to attack it felt like the controls were unresponsive. However, once I realized how attack worked in non-spirit mode, it made more sense; that is, there's a half second delay or so between click and attack because you are human (though that half second delay could be made more clear with a wind-up animation, think Dark Souls or Monster Hunter), and it's one of the main advantages of going into spirit mode because that delay is eliminated. The other advantage of spirit mode being enemies track it instead of you, however, I think it might be better if you didn't take any damage in spirit mode (or maybe it took away 1 attack instead of hp).
I got to red boss man after a few attempts but had very little health, and the lack of a checkpoint is not very motivating for me to go through again to beat him. Anyway, I liked it -- good job.