Unless the seller has to be immediately out of the mortgage I see sub-to as an excellent cost reduction strategy. just reading between the lines.. but if the bank says they will take 91k for it sounds like the mortgage is in default so a sub to might not be possible.

Doing this results in a reduction of the effective amortization to 17.08 years (205 months).  

An additional benefit of a detailed engineering-based Cost Segregation Study is that it can increase potential insurance premium savings as well as provide support for the property tax appeals process.

There are also additional costs of operating and maintaining an LLC, like separate bank accounts, annual report filings, tax filings, etc.2.

Do they have insurance and property tax reduction reports in their study?

The only additional thing that needs to happen is make sure appraiser makes note of why the value increase in as is vs previous purchase price.

As for valuation, I see no reasons why a Junior ADU would negatively impact additional housing unit.

However, our tenant turned out to be problematic—late payments, issues that led to an eviction, and additional repair costs before we could relist the property for sale.Now, after months of making mortgage payments out of pocket, we finally have an offer.

Ok it might not be 1:1 but it will add additional revenue.

I would have contacted the locksmith tech to get additional details before assuming it was tenant damage.