It sounds like maybe you're the assistant director, and now the acting head for the remainder of the year? I would be asking for the HD stipend for the rest of the year. I doubt you're getting any additional pay past that.

Offload everything you can onto your long term sub assistant. I wouldn't trust them with score study or conducting a group, but they can handle a lot of the office stuff. Daily attendance (even if you have to be the one to enter it into the computer), gathering quotes for purchase orders, bus requests. Even if they can't contribute musically, they can be in the rehearsal to help manage behavior. Who knows, maybe they can pick up enough music to help the percussion count rests and stay in time.

Do you have some student leaders who you trust to run a sectional? Your sub can supervise the kids in the other room and help manage behavior while your section leader is handling the music.

Can your MS director(s) help out? They need to be focused on the MS program first and foremost, but maybe they have some bandwidth to come help a few hours a week.

"Spoiled" below is correct that most of the country has only one band director at a school, but outside of Texas the expectations on the band are also much less. Keeping up the concert program is going to be a lot of work. If you're interested in taking over the head director job, demonstrating that you can handle it is better than any job interview.