There are a lot of possible long term things that can kill. There are not a long list of short term things I am confused over a time line now was it a short term (days) or long term (months)?

Long term I might be tempted to look at disease or chemicals I do not think air fresheners or candles over a short term and it seem unlikely to be a daily practice over months. I would look at cleaners or insecticides or temps. The OP posted hot spot surface? temps only not a full set. Dramatically low ambient will kill as well maybe not as fast but...

Has there been any use of insecticides mite treatments or anything else?

What is the disinfectant used and what cleaners?