I'm going to look into the double jump, it shouldn't be any more different than a regular jump since double jumping just forces a jump mid-air.
Edit: the floaty peak time is indeed canceled during a double jump, good observation.
The damage popups are going to be reimplemented, I also considered a DPS meter in the UI, but this might too strange and confusing for a regular player.
The heart weapon is just right next to the heart boss, I don't know if i updated it in the UI.
old savefiles shouldn't have any issues for newer versions, it might break future features but currently there are many checks when loading a savefile, so if nothing is on the save it resets to default values.
Thanks for the feedback!!