Corozal is a small town. It does not shut down in September -- hotels & restaurants will be open. In general, businesses on the mainland don't close down like on Ambergris.
Travelling between Corozal & Ambergris can be done by flight with one of the two domestic airlines or water taxi. Here's a website with some transportation info...https://belizebus.wordpress.com/
Some expats choose to live in town or nearby, but many live at Consejo, about 8 miles north of Corozal Town. There's no public transportation to Consejo (maybe that has changed), so you have to use taxis (BZ$20 each way) or rent a car. Some people hitch. Make a deal with a taxi driver to drive you around for a day.
There is an expat meeting once a month at Corozal. They meet at one of the local restaurants for a lunch/social get-together. I think it's the second Tuesday of each month, but post on the other Belize forum, and someone will confirm that. Or do a search on the forum.http://www.belizeforum.com/belize/ubbthreads.php?ubb=cfrm
As for getting around the rest of the country, flights are quick & easy. For point-to-point ground travel, transfer services are more comfortable than the buses. For you, a car rental might be best.
Other locations you should look at: Caye Caulker, San Ignacio, Placencia, Hopkins area.