1. No Thai border crossing is open
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or has ever been open around the clock. Mae Sot-Myawaddy is currently open between 630am and 630pm Thai time.
2. Normally you would be allowed to drive your car across to Myawaddy for the day, but this is currently permitted only for Thais. Only the blue book is needed. Very simple process. Only permitted to drive within an approx. 12km radius of the border though.
3. You can't spend the night in Myanmar without a visa and they're not recognizing them at the land borders at the moment. Pre-Covid, at this border you needed a visa to spend the night even if you were intent on staying in the border town of Myawaddy. The 500 Baht or US$10 pass is a day pass. Unlike some of the other land border crossings where for the same fee you could spend up to 2 weeks in Myanmar (though only in certain towns and cities near the border), Myawaddy has never allowed this. It has only ever been day pass or visa or e-visa.
4. Currently, as I have mentioned, only border bounces are permitted for farang. You can't visit Myawaddy, not even to grab a cup of milk tea near the bridge. You get your passport stamped, handed back to you and told to head straight back to Thailand.
Any accompanying Thais are permitted to enter Myawaddy for the day with the car, but my guess is they wouldn't be interested, if you're not allowed to.