You can apply for Non-O based on being a spouse to a Thai national (Spouse and Family Visa) or Non-O retirement. Both are single entry visas valid for 90 days and give you 90 days on entry. Then you can apply for 60 days family visit extension at the local immigration office. No border run for 4-5 months.
Do you mean Non-OA visa for retirement? That visa requires you to have specific insurance to apply. But other Non-O single entry visas don't require insurance.
BTW, You have a good and understanding employer who makes the paperwork as of the last day of your current stamp so that you are not overstaying even after you finish working with them. Usually, a termination letter is dated as of the last day of your work.
Your employer is basically right. You are the one who is responsible for an extension stamp on your passport. They can support you, but they cannot cancel your extension without you.
Go to cancel your work permit and extension to leave Thailand without issue. You need to check with your local immigration office and labour office to see which is the first to be cancelled. Those procedures could vary depending on the province.
-> You should be good with those (Printed eVisa Non-OA, related insurance policy, passport) in hand physically at the passport control.
Q. she will apply for a non O in Thailand as a trailing spouse/depentant - is the marrage certificate all thats required for the non O for her?
-> Well, sadly she cannot apply for her Non-O dependent on you inside Thailand. Immigration offices don't issue Non-O dependents to retirement visa holders. She must apply for her Non-O outside Thailand. Apply for her visa BEFORE you leave for Thailand through eVisa to the embassy/consulate in Canada using the information of your Non-OA.
They should issue Non-O dependent on Non-OA holders. Check their requirements.
When you two enter Thailand you will be stamped in until your insurance ends (or 365 days if shorter) and she will be stamped for 90 days.
Once in Thailand and when her stamp gets less than 30 days, go to the local immigration office to apply for her extension based on being a spouse to you as a Non-OA holder. Then she will get her extension matching your stamp.
Don't forget to get the latest requirements for her when you are in Thailand and in advance of her application.