Soil hasn’t had a stretch of dry weather for a while.

@Yehonatan Baran yeah being from 1950s this is generally from poor soil compaction when the home was originally built and has been like this for years.

The benefits of off-site fabrication are plenty: There's no inclement weather to worry about, there's more efficient sourcing and utilization of materials, less material waste in the factory, less site disturbance, less neighborhood disturbance, less soil compaction, less erosion from construction activity, better quality control, larger purchasing power, there are many more.The modular fabrication market in the United States is disturbingly inadequate compared to what's happening across Europe and Asia with the adoption of robotics and automation in their production.

Use the cold chisel to smash and remove the threshold in pieces, and remove the adhesive underneath it.

The issue is that carpet is not really a suitable material for outside of a shower, so the solution to the problem is really yours not the tenants....as in I would be hard pressed to expect that a tenant should be subject to an opinion of what is a suitable sized floor mat for outside the shower....There is a bit of a subjectivity to the discoloration i.e. if they walked inside with motor oil on their shoes, obviously that is not a 'normal' level of soiling.

I'll actually just rip it out and refinish the oak floors underneath in semigloss poly when the time comes.The property rents for $650/mo and NOI after all property taxes, insurance and all operating expenses last year was a little over $5100.The furnace and stack issues sent the shady fools that inherited it and tried to operate it as a rental running to an agent in a panic. 

As I mentioned, I have been fortunate and never had a tank removed that was leaking but was quoted the soil remediation for a leaking 550 gallon residential runs in the $11k-$13k range.

The bigger thing to check here is if its failed in any locations and causing leaks and deterioration of the structure underneath.

Debating whether or not I should hire someone to inspect this area and see if there's any standing water or moisture build up (during inspection, there was some moisture build up but I'm planning to fix that by improving the soil grade around the house - so it drains away more effectively)7) Vinyl exterior - periodically pressure wash it every few years?

If you have the land and soil, you can do a simple pipe and stone septic system and it shouldn't be more than $10,000 would be my guess.