Normally, sure.
But this is a different situation - with a guest DM that unintentionally derailed things (well, the players derailed but under the guest DM).
It merits an examination and discussion of where the group/campaign wants to go from this point.
And one, not TOO out there option, is essentially a save point. The actual DM looks at the players (or the OP if he keeps DMing) and says ok folks we've just had a couple sessions of your mirror universe murderhobo selves and how they would take on the situation. If that's how you want to continue, alright we'll go from there. But if you want to revert to your non-mirror universe versions and play it out differently, we can try that too. Thoughts?
Obviously the murderhobo comment etc. would only work for the right group and would have to be modified depending on players involved. But there are definitely options other than scrap everything and start over (though that too is an option, especially if the old DM doesn't actually want to come back for a while).