You should be doing this with any resident that violates any lease provision.If she provides ESA documentation, you need to know how to discern fake or insufficient evidence from legally-compliant evidence. 

Few things with MH that need to be assured is "Double wide" which you covered, On a permanent foundation/concrete slab, Not on axles/remove, Not moved from another property or living location, and built after June 1976 (Some investors require within 25 years). 

The most successful D class or 100% section 8 project I ever ran into was in Hattisburg MS. the original owner built about 20 little stand alone units  on the property and each was built with cinder block so very minimal sheetrock inside.. concrete floors throughout with drains in the middle ( kind of like you see bathrooms in Asia and other parts of the world . 

It will be all concrete work , no blocks. 

Is anyone out there familiar with ADA accessory rules and regs?

Build a legal rental unit with kitchen, bathroom, new windows and others, according to the design from Sokol Design-Build GroupDig out the basement and install a new concrete slab, it's a separate addition job and will cost $ 15K - $ 20K.much better than the first one (and yes both are insured and are experts in the field) I may have to GC this project myself to save some money. 

They can advise you on the specific rules and regulations that apply to your situation and help you maximize your deductions while staying compliant with tax laws.

I am assuming making it ADA compliant (wheelchair ramps, etc.) would be a prerequisite. 

Well, it's a basement, I just assumed it was concrete walls. 

I had a remediation contractor come out, and he told me the levels on the report is what he'd expect from a basement in Portland, and any amount of washing / bleaching wouldn't reduce the levels much long term as more moisture would enter the space up from the slab because concrete will always give off some moisture, and he couldn't really recommend much remediation.