I have been given permission by the county to build another home on the property, the manufactured home would be considered a guest house. 

Obv if I did that without proper permissions, it wouldn't end well for me. 

Innago, for example, permits multiple users and control over permission.

Also a buyer had mentioned to me something about a seller disclosure form to be submitted to the sellers mortgage company where they give you permission to see their mortgage info.

Get him to give you permission to enter the home and evaluate its condition.

Is this type of RE transaction permissible in TX?

You MUST have a mortgage statement so someone (her or you with her written permission) can contact the company.You can try public records, but it the grantee on the deed of trust or mortgage is "MERS" you're stuck.

She saw my T-shirt for my construction company and took a picture (with permission).

It’s illegal for a company to send you a text message if it doesn’t have your permission, barring a few exceptions.

I offer these types of tenants two solutions: 1) I will set up ACH debit for them (after they fill out a permission form) and Buildium will debit their checking account on the same day each month or I send them a paper with the paynearme barcodes and they can run to walmart or cvs or walgreens and pay at the store with paynearme.