Even with the paperwork I’d still leave the car at the border parking, savannakhet its easy to find a Tuktuk anywhere, you may have to stay at least overnight, but you can ask the immigration when you check out whether they will allow you to return early evening, be aware of fake booze, I brought back a bottle of gin from the nice duty free shop in town a couple years back for a friend, I was quite surprised that he finished up binning the bottle after pouring a glass.
There is unlikely to be any issue with your criminal convictions as you mentioned they are not serious, over 10 years convictions are often considered spent anyway depending on offense, I have been staying here on visa based on marriage, and also retirement visa and wasn’t asked for any police checks. Depending on the conviction some convictions can be removed in as little as 7 years and can also be requested to be removed particularly if you were under 18 at the time of offense, this how it used to work in Scotland anyway,
most of Africa is a breeze compared to this for visas, was in Ivory Coast during the coup and decided to take a drive to Accra during a weeks downtime, Friday morning went for the visa application was told come back Monday, however I wanted to travel there for Saturday, they said impossible (until I passed a few CFA ) they said come back in afternoon to collect,,, great drive along the coastline some cracking beaches there, just a pity the local authorities police and army had roadblocks every bridge crossing trying out their intimidation techniques
Just do exactly as you mentioned,, in and out with your Thai visa passport,, you will only be asked at checkin desk to show both as they’ll want to see your china visa and your Thai entry when you leave for china,, I’ve been in/out Thailand for 16 + years on a near monthly basis using 2 Uk passports.
For immigration
you won’t need to show 2 but anyway no issue showing 2 passports, other than occasional confusion, they cannot confiscate your passport,
I asked them on entry one time if they could switch stamps (multi reentry and extension ) to my other passport which they did no issue.
I had an issue once at a land border years ago when my wife mistakenly gave the minivan driver both my Uk passports to pass to immigration along with hers and kids PP’s on entering with a new visa in my passport 😆, was removed from minivan but resolved quickly.
I occasionally transit via Phuket on the way from Dubai to U-Tapao if I can’t get the direct flight to UTP, usually escorted by ground staff from the plane onto same bus as the other passengers then exit bus separately and escorted through to immigration, then taken up to your domestic departure gate assuming your luggage checked all the way to final destination
I’ve had same bank since more than 14 years, moved to another province and twice tried to register at the new branch, somehow my account still shows the initial province address and still get the atm charge, I gave up, hardly use cash anyway mostly the QR code so not so much an issue for me.
Edited to add ,,, I did open a joint account at the new branch,,, I but guess for the personal account I was trying to do as I would do in my home country rather than try to close account and open new was trying to change the home address, to new province possibly that’s where the issue comes from,, likely open new card account easy enough.
I seriously doubt the agents in Pattaya or Bangkok or Hua Hin will open an account for him in Kanchanaburi, more likely they will open an account for him in the locality of the agent and tell him he can use that account in Kanchanaburi which of course he can do, but will have small transaction fees I think it’s around 20 thb at atm machines for each transaction, however if he’s financially fit to live with that until he can then at a later time open an account in Kan then it might be worth it to him,,, otherwise more difficult to find but I’m sure there would be visa agents/ travel agents in Kan that may be able to assist him in some way.
probably mostly I would I go through agent is I think I would get my passport back a little faster which is important to me, as I may unexpectedly have to travel short notice,
I was just curious that I might find any issue later down the line if I did go by myself to do an extension, due to being short in money in bank a couple years earlier
I’m from UK and Never been asked for proof of funds in 16 years of near monthly in/outs, possible you may need proof of onward travel after your stay expires,, however I’ve been asked that only twice during my travels and that was by the check in staff.