Yes you are correct. 60 days 60 Aud plus extension at any immigration office for extra 30 days 1900 baht.
Technically there is not limit for the single entry visa for single year. You will know once your application gets denied. That’s probably the limit. You not gonna apply more than 5,6 visa in a year anyways.
If you thinking of keep coming back to Thailand. Might as well apply for multi entry visa. One multi entry visa will get you max 9 months in Thailand.
looks like the IO used wrong stamp when they issued you re-entry permit. You should check the stamp before leaving Thailand. Happens all the time as IO stamps the wrong date. But you case is the wrong stamp. You might have some explaining to do at the counter. Hope you get in smoothly.