Why didnt you just listen to the lady and get your M instead of going through all the trouble now? Your drivers license shows your G class expiry date.
As for the answers to your questions, they are answered above.
He did listen to the Lady - she told him he had a full year from his birthday - in December, which meant he could take the M2 Exit during the riding season.
As for extending because it expires during the winter, the applicant has some control over that, so I don't think MTO needs to build that in. In this case, by the OP's explanation, it must have expired in February or March, even April. That means he took the course late in the year at Sheridan, and waited several months before taking the AODL in to upgrade (probably because he wasn't riding - it happens all the time.) Expiry is 5 years from when you do the upgrade, so it's worth planning for in advance.
In any case, most of the answers have been provided - you'll have to do the tests, but there is no minimum time before upgrading because you've already done that. The requirement really isn't that you do the test again, but you can only upgrade from a valid licence (or recently expired). The only way to that is to do the test.
Since there are no time requirements, you should be able to do both right away. Write the M1 test inside, go out and do the M1 exit. If you did a course to get your M1, you did the MOST test. You should have no trouble with the M1 exit at a Drivetest Center because they don't even use the MOST. It's way easier.