My inspector was in the crawlspace and noted no vapor barrier or insulation under the house. 

.- If we use an RIA (registered investment advisor) it adds a layer of insulation if we were to get sued but it also adds a "mouth to feed".

Unless you want to rip up subfloors or ceilings and add insulation.

If you have very well insulated building you may be ok.

I want to hang onto this tenant as long as I can since they are literally the ideal tenant however given that they may want to utilize a smaller lease term I need to insulate for vacancy and turnover cost.

It's two bedroom, two bath, needs new windows, doors, insulation (particularly the second floor), electric, plumbing, HVAC, kitchen, and the detached garage (single car) rehabbed as well.

I'm looking for dense pack blown insulation for the front and rear walls, 2" closed cell foam for my basement w/ exposed joists and possibly doing the floor space between the garage and first floor (it's a 1950s airlite) and the attic. 

Would an underlayment I lay down myself be a better or worse option than the pre-installed underlayment or no underlayment and insulate the dropped ceiling below for noise reduction if needed?

An older building with poor insulation and a lot of air leaks will be challenging with a mini-split.

I've been told by some "old-timers" that military towns are fairly insulated from macroeconomic factors, particularly in lower-priced markets like Augusta, GA.