Can't you do the 90 days report a week earlier? Or Online-if you do it online it takes a few days to 'clear' so do it 7 days before and if its rejected you can go in and do it, before you leave.
For 3 months you wont need to leave the country if you get the TR 60 + 30 before you leave OZ. This fits snug with your plans, early on in your 60 days visit an immigration to get your additional 30 then forget about visas etc for the rest of your trip-
You all go in a van together, they will assist you, from CM prolly go to Mai Sai. I did it with a shop front agent near Mikes Bar old city on Moon Muaeung - may not be there now....