In the event that you do have a true squatter its a fairly standard eviction process which is unfortunate but does happen in this business.On a preventative note if you have a vacant unit you can put in a security system that uses cellular and you will get notification if anyone comes in. 

Is it possible to find a single-family house for around 100k that is compatible with Section 8 standards? 

While commissions are negotiable, industry standard today is 6% of the gross base rents for the first 5 years of a lease term and 3% for any years after that split 50/50 between the landlord's broker and the tenant's broker.

Concept2 seems to be 'gold standard' out there.

Yes and all the lawyers will collect their standard fees. 

The loan caps vary across lenders, across experience buckets, across asset valuations but 70% - 75% is standard.9mo note at 12.99 i/o - Most say "12" months and then bake in a fee if it goes longer than 9 months.  12.99 is high. 

With standard ceilings, and single-story I would expect to spend about $100k for basic turnkey on that addition with a hungry builder. 

Unfortunately, many owners & PMCs maintain Class C & D properties to Class A & B standards. 

I don't know if this has been standard across the board, but in the few that I have looked at this also seems to be the case.

But when one of the parties is licensed by a governmental board of some sort, the board can subject the licensed individual to higher standards behavior, and they can do this largely at will and without the higher thresholds of most criminal laws.