Washer and toilet water backing up into shower. Can’t change plumbing. What can I do?

Renting a tear down, landlord does next to nothing, house will be torn down. Water drainage from house is not working, also not going to be redone. So the problem is drainage for 3 showers 4 toilets and the washing machine. 1. Downstairs issue- washer drains into shower basin. Have tried various drains to prevent this but the water comes up with force no drain cover barrier or plug prevents this, Pretty sure it is also on the septic line because is stinks more that laundry water would, also make major mold issue. 2. All 3 toilets upstairs all have flush issues and clog often/ don't seem to have the water pressure to drain fully with a flush. I have been told that the main drain for the house is original, 1942,and is the root of the problems with drainage. The access to that pipe is outside under the concrete 5 step stairs. The access to pipes that flow to main drain from all bathrooms is also not accessible with out major demo and renovation of 2 bathrooms and 2 bedroom on 2 floors.

I just want to have a bit of a win with the shower backing up issue…I have been fighting mold for 2 years and I just tiled the shower basin…but I have to find a way to keep stank water from back feeding into there. Other tips that may help keep this old house flowing are appreciated. Please don't bother telling me that the pipes need to be fixed because its not my house and the landlord doesn't care.