They just informed me that their son has been diagnosed with autism and the doctor gave them a letter/prescription advising that he have a "comfort dog". 

They get a prescription for the service animal and then never go back to the doctor.4.

The exam is a license that allows you to operate in a certain market similar to licenses by securities brokers, attorneys, physicians, plumbers, barbers and the like.

The first resident would need to provide documentation from their physician stating the medical need for this accommodation. 

If they can prove they have used your land, had their fence there, etc. with no objections, then they may claim prescriptive easement. 

I’ve been working as an independent contractor physician since Nov 2015 and had two years of income tax returns and had to submit recent pay stubs and bank account statements to qualify for the FHA loan.  

The agreement can be as simple as to say the 'property owner A' and property owner 'B' hereby acknowledge the proximity of their respective driveways and affirm that neither property has any easement right/necessity over the other.The concern here from a lender/title perspective is the ability to claim a prescriptive easement right.

Exceedingly long-story made short: I moved to west Texas 5 years ago to pursue my big, hairy, audacious goal of becoming a physician.

One word of caution: I wouldn't advertise that you are a physician/surgeon.

Soon to graduate from Physician Assistant school with zero assets and tons of liability so I have some work to do!