Tell them you plan on doing a land border crossing to Laos or Malaysia or Myanmar or Cambodia and an onwards ticket is not necessary. Book a fully refundable or zero deposit hotel in either of those countries as further evidence.
standard certificate issued by medical practitioner for purpose of obtaining DL.They do it multiple times a day so it's not uncommon. They will know what you require when you ask at reception. Cost 200-500 baht depending upon establishment.
no visa. Just entry stamp but do it straight away not towards the expiration date. Medical certificate from local public hospital. TM30 from your hotel. Home country licence and IDP. Job done. No agent.