Your stamp for dtv is 180 days if not you need to go back to airport immigration and get it corrected. Makes no difference what date you put to depart on your tdac. You can put any date or leave it blank. You don't need to show a return flight with a dtv.
Ho Chi Minh City, formerly Saigon..the latter name still used by many including locals . Great City incredibly cheap, hotels,food,restaurants, grab taxi only ยฃ1 approx for most City trips.
Fly for a nice break for a few days, enjoy, keep life simple, so many nice places to visit within an hour of Bkk, DTV is to be enjoyed, 5 yrs soon passes
allow 2 weeks, HCMC is great, just avoid any Thai public holidays, as that could extend the time. If you provide everything and are not asked to submit further documentation you might get notified within 5-7 days. Just ensure you don't leave the country you applied at during the process!