I was told years ago from my Thai bank that any value over 10,000 USD is liable to a higher risk of investigation for money laundering, meaning legit transactions could be delayed, I’m not suggesting your payment has been delayed nor am I suggesting any illegality going on here but if you’ve made regular transfers above the 10k usd then maybe they’ve been monitoring your transactions and thus caused issues.
Usually they check, but I’ve been asked once when using a one way ticket if I’d planned on doing an extension once there, (which I did ) this following the check in desk checking through my passport pages and I guess them seeing mostly yearly extensions or yearly multi entry visas with plenty in out stamps, so you may get away with it but best have a dummy onward ticket just to be sure.
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.