You should have a stamp from your embassy at the front basically saying transfer Visa to new passport. If you have time before traveling go to your local immigration department and swap it to the new. 5 minutes and done
The form doesn't have multi, put in brackets (multi), pages 5000b. If all papers correct pick up next day. Make sure you have original marriage cert with you
Show your paper trail. In Chiang Mai isn't a problem. True they probably give up after the 3rd set of bank statements for the same transaction but it is possible
I use orbit remit, at immigration i show my Australian bank account with the exact same funds transferred to orbit remit, then to my Thai account. Yes the Thai account says internal transfer, but if you show the paper trail at least in CM it works