After an hour of conversation, he agreed to our cash-for-keys arrangement, provided he had two weeks to get ready.

Texas home equity laws are different then the other 49 states and do NOT allow for these arrangements.

Without a written record of this arrangement, is there a way to claim any compensation for those renovation costs, or would that be difficult to prove in court?

Just make sure the arrangement is clearly stated in the lease agreement.

More often, I see people in Chicago using the MTR strategy to side step the STR regulations.I've considered buying a SFH with an in-law arrangement in Chicago to use as an STR because to me that seems like the best way to employ the strategy.

As a listing agent, I pre-arrange a specific weekly or biweekly time window with existing tenants and arrange all showings within that timeframe, with POF/pre-approval required with offers.

Ultimately, clarify with each broker upfront about your intentions to avoid misunderstandings and ensure they’re comfortable with the arrangement.

But you can arrange to get paid in the interim. 

I offered tickets to XCARET and received commissions for arranging rental cars for my clients.

@Alicia KulurisI know I'll get slammed for even mentioning this but what about an arbitrage arrangement?