Hello, thanks for checking this game out :)
This is an error the only happened on the android version due to me accidentally leaving a line of code in that tried to call the next day which wasn't finished yet. The demo was supposed to end there anyway, so you haven't missed any content. The healer class will be available in the next update, whenever I can get it finished :) Hopefully it won't be too much longer.
As far as I know, the saves are stored in a separate renpy folder somewhere on your hardrive so any time you load up the game it should pull from that folder so if you were to download the newest version, it should still pull from the saves in the renpy folder (if you are playing on android, I'm not as familiar with the save system storage there, but should be relatively similar).
Though, the first few updates to the game will require you to restart the game from the beginning because of how adding variable and assets to earlier portions work. Once the game is further along, you should be able to play the game starting from a save file without issue.