There was a recent case on here where a Thai with dual passports showed both on arrival and the IO stamped the foreign passport for 30 days. She was going to have to leave and return to not be on overstay. Don't give them the chance to stuff it up.
Depends where you are. In Hua Hin they don't care. I just renewed my retirement extension and asked about updating my TM30 and she said if I'm still staying at the same place, no need to report. I'd only been back a few weeks.
The 4 diseases that need to be checked against are extremely rare or non existent in advanced western countries. My GP in Australia just signed it with no bloodwork. But I've been seeing him for many years. YMMV