Does the tenant have a baby or could the blockage be the result of someone who lived there before and it just shifted and blocked the pipes now? 

I had to go in every two weeks to change the filter now that air is moving non stop.Fast forward a few months of quiet time, there is a main line drain blockage that happened on a Tuesday evening. 

If low water pressure suggests a blockage or corrosion, is it typically a straightforward fix—scope the line, replace the offending section, and move on at modest cost—or does it more often signal hidden piping issues behind walls that demand full repipes costing several thousand dollars?

“can’t flush” calls.Before replacing, make sure it’s not a deeper plumbing issue—like a partial main line blockage or improper slope.

Additionally, there were blockages that prohibited the kitchen sink, bathroom sink and toilet from operating a full capacity.

The plumber noted that despite the faucet being left on, there had to be a blockage to cause water to significantly overflow as it did, which is what should be reported to the insurance company.But, as I mentioned in the intro, there are a few issues here.

Airbnb rejected claim stating it could have been in toilet previous to guest that experienced the blockage (no way, nothing but liquid was going past a vape in the s trap).  

Not including septic of course because that;s an easier run.I would do a sewer scope even if just to know if/where there were branch lines or moderate blockages.....or even the bellies.

Sewer Cabling/Drain Cleaning: When there's a blockage, usually paid by the owner unless the tenant caused it.

There are openings in the upper west exterior walls where electrical conduit was removed.10.The stucco in these areas has not been repaired to prevent moisture penetration and deterioration.11.The front unit has issues flushing the toilet, which was confirmed with a tenant, so camera scoping of the drain pipes was performed that revealed damages and blockages in the pipes.12.Deferred maintenance on multiple items: cracks around windows and pipes, gas pipes are not supported, pilot lights in heater units were not lit, a water heater unit has no ventilation.I hired a plumber to scope the pipes (please see his report in the invoice attached and photos); a roofing company was hired to assess the damage to the roof above the middle unit only (please see proposed work sheet); a general contractor has inspected the settlement of the wall in the second unit due to the damaged frame.