Other issues with adding an ADU:- in many CA jurisdiction the existing home becomes rent controlled.  

If you can't keep control of your personal finances, you are highly unlikely to succeed in real estate investing.

You will have total control over your destiny.

This approach allowed us to take ownership and quickly resell the property, providing greater control and flexibility.

You are allowed control over who pays what costs through control panels within the program… so you can control who pays the fee to screen a tenant and how much you charge for that service as well.  

Consider checkbook control.   

If all else fails, i'd look at the local laws to see what type of escalation is possible and have them take it from there.It's unfortunate for your tenant to be going through this but ultimately the situation seems to be out of your control considering you've already sought out a resolution on their behalf.

I abhor rent control and that seems to be the direction they're going.

For Commercial RE, we use Crexi, Loopnet and Brokers that control the markets. 

Hard Money lenders prefer first lien positions during the rehab portion due to the enhanced security, control, and financial stability it offers.