I don't understand all of the save scumming that people seem to do. The whole point of table top DND is that if you fail a roll or make a "wrong" decision, that shapes the narrative of your story.
You haven't "failed a quest" you have opened up a new story path.
You aren't going to ask your DM to forget the last hour of play irl, why would you do anything different here
I don't know if you want to use mods or not, but I'm pretty sure I saw an increase party size mod on nexusmods the other day