It should be okay, but no guarantee for that. Normally the people fly in/out at Phuket, not at Bangkok. When you come? It needs normally some days before changes come in effect.
No, if they cancel "test&go" then it will be "Sandbox/Blue Zone". Seven days in a SHA+ hotel, you are free to travel around at Phuket, but have to sleep in your booked hotel. Also the second test will be RT-PCR, not ATK. After that you are free to leave Phuket (if the test is negative).
There are no "border runs" at the moment, all land borders are still closed. Nong Khai seems to open on 24th., but no details are known yet. My guess is, that the "traditional" border run by land will not be possible for a long time, maybe never again.