It's the airlines that are the gatekeepers not Thai immigration. The fact that you're going to get a retirement visa 'means' you don't need a return ticket etc. If its Turkish airlines it should be ok, explain to them you were advised not to get a return b/c you are retiring in Thailand.
Most Visa companies out of BKK or Pattaya use Ban Laem crossing. Best to use a visa agent company. But yes you can do what you want you want to do. Avoid Poi Pet crossing.
You don't need the re-entry permit. Just come in and out on Visa Exempts. The re-entry permit is essentially for longer termers not Tourists. Unless you're a habitual abuser of Visa Exempts you can do a couple back to back without issue.
Your making it way more complicated. No visa required beforehand. Come in on the 30 day Visa Exemption which you can extend for another 30 days, giving you 60 days total. Go to Brisbane then come back and get another 30 day Visa Exemption. This is the simplest way for what you want to do.