Awesome game, I'm glad Castlevania Granddad can rewind time because those deaths sure look painful.
I had no major issues while playing. I guess the MC was a bit too grabby with ledges at times and that dash maneuver probably needs to get remapped to something more deliberate because at least I tend to tap the directional movements to make small adjustments rather than performing a long jump. Also, I found this bug but don't worry, I was fine afterwards.
I also think the Mario level was pushing it a little bit in terms of difficulty, but that's fine because I found it hilarious. On the topic of the final boss, think the pile of bones in the starting area made it clear that skulls can rewind you, but I believe it's likely players may miss it so perhaps you should make one drop from the ceiling or something instead