TBF the rules aren't changing constantly and there's nothing stopping them pulling the DTV. In fact it's probably more likely they'd pull the DTV rather than change the 'retirement visa'
You dont need to leave for 5 days. It's a multi-entry visa, you can come and go as you like for the duration of the visa with each entry getting you stamped in for 180 days
Why not just add 'extention of stay'? Surely it'll just add confusion to people by selecting the base of the extention, as if there isn't enough already over visas and extentions!
Probably wont be an issue for Thai Immigration but it's boarding your flight that you'll have problems. The vast majority of long haul flights/carriers require you to have return or onward travel within the allowed entry to your final destination, ie in your case 60 days.