Guess I would delete all issues except for the most critical bugs and any remaining critical mvp features, and then I would say that from that point on you can't add any feature requests without completing something - that is you can only add a feature ticket after completing a bug or other feature ticket.... for sprints I'd say 3 to 1 bug to feature ratio and no more than 1 feature at a time in progress / in sprint at any time.
Then if normalcy returns I'd ease up. I'd also make sure there's one lead dev in charge long term on the project instead of just having whoever has "some time to do it."
Not sure if this is ideal, which is why I'm looking forward to your answer.