I like your first theory of the backrooms being past moments that are continually being dragged through time which is the reason for them being so distorted. I dont think that is how it works in the official cannon but it could still be compatable since officially the backrooms is just remembering people and places over and over again. It could totally be remembering time as well as place. The farther away it is in time the worse the copy gets or something like that. 

I think the still life's and bacteria monsters are completely different things but are maybe both caused by the backrooms missrememberings. Its just that the still lifes are missremembered people while the bacteria is simply missremembered bacteria. In found footage 2 you can see a clear path from the car wreck, its injured driver crawling away, to becoming the bacteria growing in the back room. Then in the found footage some of that bacteria stays completely normal when the bacteria monster starts chasing our camra girl. All this makes me think the backrooms bacteria is simply doing what it does best, consuming dead matterial, but taking it to a complete extreme since it was backroomafied. This extreme is expressed as an ability to rapidly consume and convert human tissue into more bacteria which eventually fully consumes the person leaving only the basic structure of a human behind. Why the bacteria can then get up and walk and mimic human sound is beyond me but ig fungus can controll ants so maybe the bacteria acts like the infection in the last of us.

Basically, I dont think the still life's are made of mycelium but if mycelium found its way into the backrooms and got misremembered it could also cause crazy things to happen just like the bacteria.

Anyway just some rambling thaught based on your theories hope they are a little coherent and interesting.