I've been on BP for several years and feel like I have established a legitimate reputation.   

where we were asked to explain what we do, show business licenses and corporate documents to prove that we're a legitimate business AND a completed contract to demonstrate that we actually  do what we say  we do.  

You are right, a lot of new people that go at it recklessly much things up for the legitimate people.  

-Is this a legitimate claim or scare tactic?

Any legitimate self-employment activity (including being real estate agent) would qualify you for Solo 401k. 

Part 2...How long does it need to be a STR in order to legitimately claim accelerated bonus depreciation?

Vet the person, if you have a legitimate reason then do not sell to them.

It's hard to know which are legitimate questions to focus on answering.