Get both passports, leave AU on the Aussie PP, enter Thailand on the Thai PP, you should get a Thai birth Certif from the Thai Consulate ....I did this for my son, born in Dubai ..... no worries mate
Thai & US passports are no poblem, use the Thai PP going in/out of Thailand, after that you use the same passport exiting as entering for any country you are visiting .......
Yes, the Kor Roh 3 is obtained at the local Ampur, it certifies that you are still married, costs about 30 baht .... again it has to be dated close to the application date, only takes about 20-30 minutes depending on the crowd ...
Also Kor Roh 3 and TM30 ..... no insurance required. Bank Statement no more than 7 days old, Bankbook updated that day, with copies of eveything, signed in blue ink ...... in Chiang Mai at least, 4Weeks ago