@Ivan ***********
Those requirements are not only for extensions, they are in place for each stay and just get checked at approval of visa, apply for extension, sometimes at entry of the country and might be checked any moment you get seriously involved with Thai police.
I know returning on an extension is different from entering the first time on single entry visa or each time on a multi entry visa, but call that a fundamental difference is a bit over the top, since most requirements are exactly the same which makes sense, because you always strart your stay with the requirements of the visa, and with the extension you just continue that same stay.
DTV is multy entry, that means you get a new stay each time you enter, but I don't see that as totally different, at least not for requirements.
The main difference is that a single entry visa becomes invalid the moment you enter the country and a multiple entry visa stays valid the whole period.
When you are in the country you are always on a stay represented by an entry stamp legally, not on the visa itself technically.