If they want to pay $1,300/mo as is, saving you the Reno cost.

When you took in the increased cost of the build how did you arrive at that percentage?

In southern CA, even garage conversion ADU additions are typically costing more than the value added.  

It is your first year when you get the major tax savings windfall due to cost segregation and bonus depreciation.

2.What are the best and most cost-effective solutions for managing this potential long-term issue?

Ignore the $1,250 per month because that's your cost of housing, if you didn't pay that on the home you'd have paid that in rent so that's a non factor. 

We had a discussion about the costs I have endured and the low ball offer to cover expenses for going 5 months past contract deadline but haven't heard back. 

But I would charge the extra 3% to cover my costs.

Point is the above comments there are many other more effective ways to meet the FINCEN stated objectives for tax evasion, money laundering and terrorism.   

Here are some indicators to help you assess a buyer's seriousness:Questions: Serious buyers will ask detailed questions about the property, such as maintenance costs, neighborhood amenities, and property taxesStrong Offer: If a buyer makes a strong offer quickly, it often indicates a high level of interest and intent to purchasePrivate Buyers: If approached by a private buyer, ensure they provide a Letter of Intent (LOI) and proof of funds.