Do you have concrete assurances that admin will not repeat the problematic scheduling? If not, I would consider moving to another school.

What is the instrumentation of your possible 4 brass players? Are there strong WW sections with motivated students who could switch to brass? It's not fun, but possible with careful guidance and programming (especially if you start now in anticipation). I've had saxophone players become outstanding tubist in the past to fill a need.

I would be cautious about approaching students who have left in the past. You know each situation better than I do but in my experience it causes more problems than it solves.

A band of any size/instrumentation can learn music and improve. Worst case take a year off UIL to rebuild.