Now if you’re looking at 80 year old houses with leaky/cracked basements to shifting soil, I can’t help you there. 

Poured or block foundation, no major cracks or visible settling.2. 2 bedrooms, 1 bath, +/- 1000 ft2, single soil stack construction.3.

@Joe AlfordThat looks more like a soil issue that caused the foundation issues.

Even if she was in bed I'd imagine the body fluid and decomposition liquids would have dripped onto the flooring underneath the bed and gotten into the joists and such. 

The foundation is on sandy soil.

Do the sewer pipes have to be replaced underground underneath the structure.

Only soil indexes were too high , whatever that means.This concerns me deeply.

I would suggest a visit in there to see how things look from underneath, especially if a leak is suspected.

What happens is water seeks the path of least resistance, so a trench creates a soil conduit where the old and new dirt have differing density, and water tends to follow the old trench.

The odor goes inside the walls, the attic, underneath the building, inside the furnace, heat ducts, electrical conduits and even after cleaning everything and painting there are times you can still smell the odor.