Yes. Even now, most land borders require a TM6 to be filled out on paper. If it becomes mandatory to do one online on May 1, then unfortunately, even air arrivals will be affected. In fact, I suspect like Cambodia, they'll implement the online TM6 requirement for air arrivals first then for land ones later.
They won't ask you about 90 day reporting at airports or land border crossings, but they will put a 90 day reporting slip reminder into your passport.
It doesn't matter to immigration at the border whether you've done it or not, as they don't handle these matters.
A different story in China - or Beijing at least. Anyone who doesn't register their accommodation with the authorities (similar to the TM30) may get denied entry on their next re-entry and is ineligible for the 7 day visa waiver for transit passengers traveling via China, en-route to 3rd countries.
You don't and can't get your DTV transferred to a new passport. Bring both passports with you.
Apparently, if it's an e-visa, then there is no need to travel with your old passport but you should bring a photocopy of the old passport along with the e-visa paper. If it's a visa sticker, bring along both passports throughout the duration of the DTV.
Nothing special really. For now, if you enter by air and you have a visa sticker, make sure to show that to the officer. If it's an e-visa, print it out.
At a land border, fill out a TM6 form (the paper version) and write your visa number on there. The officer will find your visa (but do present the paper version, if it's an e-visa).
Some land borders temporarily exempt travelers from having to fill in a TM6 including Sadao, Nong Khai, Mukdahan and Aranyaprathet (including Ban Khao Din, which is under their jurisdiction). At these borders, show the officer your visa so they don't accidentally stamp you in on visa exempt.