The other avenue I'm really curious about is if I can take over the bank's unlawful detainer case.  

Regardless of whether they file a homestead or not, there are today over 70,000 Texas residents who owner-occupy their multi-family properties...it appears none of them can take cash out of the free-and-clear property they live in---ridiculous, an unlawful restraint on alienation, a handicap to duplex citizen-owners, and a sad reflection on the intelligence of our Texas legislature and mortgage lobby here. 

Unless, in any state, your vendee objects and slams an unlawful foreclosure/eviction on you and takes you to court over the matter, depends on what the default might be.

However, in some states it is also unlawful to advertise real estate for sale if you have no equitable ownership interest in the property or unless you are in the newspaper business.

This guy had a lawyer contact me telling me it was unlawful for me to hold his security due to motor vehicle damage.

My reading of this makes me think that we would not have to go through with an Unlawful Detainer Hearing and could go straight to having the Sheriff execute the writ of possession with a lockout.

An additional google search by attorney uncovered the February 28th judgement of case number SC12-3727 where Hetsler Mediation and Valuation was prosecuted by the FL State Bar for unlawfully practicing law in the state of FL.  

(a) It is unlawful for a person or entity other than the transferor to engage in, promise to engage in, arrange, offer, promote, solicit, assist with, or carry out a foreclosure rescue transaction for financial gain or with the expectation of financial gain, unless prior to or at the time of transfer, the transferee pays the transferor at least fifty percent (50%) of the fair market value of the property as determined by a licensed appraiser.

It shall be unlawful for any person, corporation, partnership or limited liability company to engage inany business, occupation or activity listed in subsection A without first obtaining a license as prescribed inthis chapter and otherwise complying with the provisions of this chapter.C.

If you are absolutely certain that they are not an heir nor have any standing, hire an attorney and file a motion for unlawful ejectment.