those have been halted for months already. No one knows when they'll resume. The border with Cambodia is closed due to the conflict. They've been doing overnight drives to Laos lately. I'd much rather go to Bangkok and get a flight than spend 18-24 hours in a van.
Yes. Though Hua Hin isn't exactly well situation for border bounces. Closest land border would probably be the boat crossing into Myanmar at Ranong/Kawthoung. Probably easier for him to fly somewhere especially if he wants to take a mini-vacation. Both Kualar Lumpur and Singapore have visa-free entry. Taiwan, Hong Kong, and even Korea and Japan aren't too far off.
You would need to check with your immigration office, as each immigration office has their own policy on this.
Generally, most offices require a new TM30 when you return from outside the country. Technically/legally it's required within 24 hours. But realistically, when you can get around to it before you go to immigration for something, it's fine.
An METV does work that way, but he could also apply for a 30-day extension each time, meaning 90 days + 90 days.
Although if your friend has not been in Thailand recently, and will not be staying beyond 6 months, he would probably be fine just arriving visa exempt (60 days) and then applying for the 30 day tourist extension. Leave Thailand, go somewhere for a few days (not actually required, he could return immediately, but why not enjoy the region if you're here already?) and then when he returns he gets another 60 day visa exempt stamp, +30 day extension before returning home.
Visa exempt is not a good option for staying long-term, as you're likely go run afoul of immigration, but if it's just 2 entries, without a long history of being in Thailand and not trying to stay too long, it's probably okay.
Each embassy has their own requirements about what is required for a dependent. You need to check with the embassy you'll be applying at.
But no, she cannot go to Thailand and convert to DTV. That visa is only available from a Thai embassy, which means she must physically be in the jurisdiction of that embassy in order to apply.
If they check and see that she's in Thailand or has entered Thailand during the application period, her visa application can be cancelled or her DTV can be revoked.