You can re-activate the scene with Altoflix by replacing the two story.* files in the saved game folder. This may cause problems with other quests in the village that you have partly done already, but would still be preferable to starting over.
Some quest information can be stored in the data.000 file (which keeps track of your characters' stats and all NPC's status and location), which can not really be swapped or reset. Hopefully (most likely) just resetting the story files will be enough to eliminate any problems from crossing the lava too early.
[*] Copy the 2 story.* files from
my save (at the beginning of the imp village) into your latest save (maybe make a copy of that folder first, just in case).
[*] Talk to Altoflix in the north of the village, then the chieftain's son to get the quest to find the alchemist, talk to the alchemist once (or leave that until later).
[*] Continue with the game.
Complete all other quests before giving the ingredients to the alchemist.