How does the visa exemption entry work for my 45-day allowance in Thailand?

I think you're mixing up a couple things it works like this

IF you enter thailand on a free visa exempt entry stamp on or after Oct 1st you'll get stamped in for 45 days, you can go to the immigration office and extend that ONCE for 30 more days for 1900baht <- THEN you're done and you have to leave.

You can go to the border, stamp out of thailand, stamp in and out of another country and stamp right back into thailand to get a NEW visa exempt entry (which will be 45 days if you enter again before the program runs out on Mar 31st and goes back to 30 days after that) AND you can extend that ONE time for 30 more days at the immigration office for 1900baht.

You are allowed to enter the country on a visa exempt stamp by land TWO times in a calendar year, and technically there is no limit to the air entries (BUT you're gonna get called out after a few for trying to live here on visa exempt entry stamps)