We watch the HUD Fair Market rent for the area, and that is still trending up, though in some past years there was a decline in the Fair Market Rent, which is the Median Rent with 50% above and 50% below that figure. 

If the market declines just realize you are stuck in that property.

   •   Part of the county shows moderate decline in Zillow home value index (~-2.6% over past year) depending on sub-areas.

The property value has declined to around $430,000, and I owe $446,153.I’m trying to figure out options for getting out from under this investment quickly without dumping more money in, or whether there’s a realistic path to stabilize and turn it around.

So, the value of in-person showings continues to decline with an increasing percentage of prospects actually preferring to avoid the perceived sales pressure. - Biggest efficiency savings is from last minute reschedules, prospects running late & no-shows.- Security is the big challenge to avoid theft, vandalism, squatters and scams.- Security options include ID verification, geo-fencing of prospect cell phones and video call verification.

I can’t believe you found one half empty unless it’s a declining neighborhood. 

The population of the MSA isn't slated to rise, but that doesn't mean there aren't tens of thousands of people who have succeeded by investing in this market since the population began declining 70 years ago. 

Beside the added revenues, the neighborhood has gone into decline, retail value dropping.

It showed in some cities (rare but there is at least one per the study in southern Ca) that the property value declines  by adding an ADU.There are a lot of reasons adding an ADU may be a poor investment option.  

And then as liquidity has dried up, volume has declined.