Solo attempts shouldn't be doable, which is a bug I probably introduced myself somewhere.
As for leaving, I suspect this is because damages is calculated before retreats, and this translates out of the dungeon. For what it's worth, no dungeon floor spawns empty, there are ALWAYS monsters. Regardless, this is common enough of a complaint that I'm considering it an issue, and will be looking at possible resolutions. One that comes to mind is making it so on the first level you take 0 damage if leaving, and then add a certain percent for each additional floor.