It's a dumb assumption. Firstly, maybe his parents already came to visit him. Secondly, maybe he went to Vietnam on business. Thirdly, I don't know his age but assuming he's older (by the sounds of it, he's probably a bit younger but let's play devil's advocate here) then his parents might not be around anymore. Or he doesn't have a good relationship with them. Or perhaps they live in Thailand or Vietnam. The possibilities are endless.
It's not. "Employment prohibited" means employment prohibited. There is nothing to prevent anyone from working remotely on any visa. Just that the DTV is best for those staying long term in Thailand, who are remote workers, but you could in theory be on another visa and work remotely too.
You can't apply online if you're on a DTV and you've just entered or re-entered the country. After EACH re-entry you must either mail in your report or do it in person. Only the SECOND time can it be done online but for most DTV holders, you'll either be leaving the country for a border run by then or doing an extension of stay (so you might as well do your 90 day report at the same time). Thus only at the 270 day mark would you be able to do an online 90 day report for the first and ONLY time. The next time you can do it online will be 1 year later, assuming the same circumstances.
I did a TM30 and a few minutes later was at immigration doing my 90 day report. It was quick and easy. Previously, I only ever had an agent do my 90 day reports for me, so this was the first time I've done one myself. It took me 5 minutes. I spent more time parking my car inside immigration and walking towards the building where I had to file my report.
Well, all I know is that the OX is available through some consulates only and that I understand there is a health insurance requirement. Other than that, a 5 year stamp and just the need to make 90 day reports but otherwise having no other interactions with immigration (my understanding is, it's also a multiple entry visa) and considered a long term visa too, is far superior to the DTV, for those aged 50 and over. This is assuming you're able to apply for it, in your home country or another location where it's offered. I understand it is only available to a small number of nationalities such as Australians, Americans and a few western European countries.
The DTV is fine for those with existing Thai bank accounts who come and go every few weeks or months, have businesses abroad and have a need to travel to take care of those companies while spending their time living and working in Thailand remotely (likely being married to Thais).
The 90 day reporting is very easy to do. I did a TM30 and immediately went to immigration with the receipt and handed my passport in for the 90 day report. Noticed there was a 90 day form, but that was after I had already submitted it and the clerk didn't ask me to fill out the form. I was in and out of immigration in less than 5 minutes. Took me longer to park my car inside the immigration parking area.
The OX retirement visa is the best - you get stamped in for 5 years and have many more advantages over a DTV holder, who is simply a long stay tourist who receives almost no advantages over a visa exempt visitor.