ATC Supervisor reported miscommunication with NAVAID Technicians resulted with the non-availability of an ILS that had an aircraft attempting to intercept. The aircraft descended towards terrain. The Tower sent the aircraft around and was able to land on the second approach.

Narrative

I was working OS (Operations Supervisor) combined with Ground. ZZZ Airport in support of Flight Check; was attempting to work on the ILS to runway XX (Non-Fed ILS and Techs). When the facility opened; the weather was projected to be good enough for the outage; with no immediate inbounds; ZZZ left the ILS on XX since they would be working on it shortly. In the next hour; the weather deteriorated enough that we needed the ILS for the first arrivals of the morning. Due to the low ceilings; ZZZ ATCT and ZZZ [Center] Area 1 denied the outage to retain the ILS approaches. ZZZ's equipment can only have one ILS on at a time; and it was left on runway XX overnight for the projected weather. ZZZ attempted to switch the ILS to runway XY when Aircraft X was inbounded from ZZZ [Center]. However; the control panel to switch to ILSXY would not allow us to switch the ILS approaches; it appeared to be in a 'locked' status; resulting on the ILS XX being stuck on; not ILSXY.Aircraft X was observed going through final; turning inbound approx. 1/2 mile north of course and descending to 1600'. and descending below the MVA before calling ZZZ ATCT. ZZZ tower asked if they were receiving the ILS Signal; with an answer in the affirmative; despite the control panel not showing XY functional. It is believed that Aircraft X was receiving the Back-Course guidance from the ILS XX. Aircraft X was sent around out of precaution; and coordinated with ZZZ [Center].I was talking with the local FAA TechOps during this evolution trying to get the ILS to switch from the equipment room. They advised that the 'remote' to switch the systems was disabled. They contacted the Non-Fed Tech that was at the ILSXX glide slope and determined that the Non-Fed Tech had disabled the remote from there; despite being denied the ILS XX outage. FAA Techs told them to re-enable the remote. ZZZ was then able to change to the appropriate ILS approach; and resumed normal operations from.Suggestion: 1) ZZZ Airport and their technicians need a better understanding that they can not start performing work on something before an outage is approved. 2) The Non-Fed techicians need to plan to have the work done before flight check arrives; and not playing catch up the morning of. While this is opinion; I got the impression they were in a rush to complete the work before Flight Check arrived; and with the bad weather; they tried to do things without approval.3) Each ILS needs to be on their own frequencies.When the ILS to XX was being designed and built last year; I raised the question why it was being built with an interlock system; and not two independent systems that would use two separate frequencies. The answer was always 'engineering says it has to be this way.' Having two separate frequencies upon installation would have 100% eliminated this problem. It would also alleviate the issues of only having 1 ILS available at night when the tower is closed. If the wind changes beyond what is forecasted; planes could potentially be forced to use a down-wind ILS; risking a go-around; or divert. When one ILS needs maintenance; we have to shut both off; leaving us with RNAV only approaches.