@Chris ********************
when we did this application several times the "proof of reasons to return" were separate documents, and not part of the letter of support. We provided her employment details, actually proof of business ownership, evidence of her financial assets (bank statements etc).
It is a fine line between providing enough information to support the application, and looking like you are pleading a defence for something you are not going to do. The UK visa processing, is in my experience, less subjective than some other countries we can think of. It really is a tick list of documents and actually discourged to provide things they don't ask for, imo.
Letter of support or invitation needs only the things I mentioned, and the subject for assessment is actually the invitees own status and independent financial standing. Like you say, proof permanent employment in Thailand is
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th of the way.
Good luck to all those honest couples that try and deserve approvals without challenge.