Sounds like you have left your employer, which means your work permit has to be returned to DWP if you havent found another employer within the 90 days and can update the WP and provide paperwork for the extension. Failure to return the WP would make it very difficult to obtain a new one in the future.
on the premis that both your metv and dtv are e-visa's, yes you should be OK next time you enter, as you will present the DTV pdf to activate it instead of the metv pdf. If the METV is not e-visa then you may have questions at imigration as you will be presenting 2 visas at the same time.
and you can ignore any automatic emails telling you to report until you've been back in Thailand about 75 days. They don't account for you being out of country.
the early coms before live date said one extension in the lifetime, post live date the coms now say one extension per entry. But you still have to leave and reenter once a year on top of an extension to get a full year.
i'm on my 2nd 5yr licence, and have never had to produce my passport. I do have to produce a permanent address, which you need to get from immigration, they only issue those for people on long term visa's, ie 90 day+, a DTV is longer than 90, so you can go to IM and get one. The gov are giving 180 days, they wont put people in a position where you are illegaly driving after 90 on an IDP and home licence as you cant get a Thai licence.
You dont need a visa directly for driving licence as such. You need proof of residency which is obtained from immigration. Immigration will issue these if your on a long term visa or extension of 1, a DTV should qualify as its greater than 90 days and you have a permanent address.
i suspect he made the mistake of going to the VOA desk first, rather than going directly to passport control desk. A lot of first timers entitled to exempt entry do that, myself included first time. Usually they get sent directly to passport control as their country isnt on the VOA list. Unfortunatly for your friend, his is, and the VOA team didn't enlighten him that he didnt really need it.