Does a Non-O visa expire and what is the activation time frame before entering Thailand?

If you enter on a Non-O when unmarried then the first extension will need to be for the same reason i.e. retirement/over 50. For the second extension, if you are then married, you can change the reason for the extension.

as long as you are not married, you cannot apply for the Non-O marriage visa. Your INTENTION to get married doesn't cut any ice

Look at how long it takes to get the certificate your free to marry from your embassy. I had to wait 6 weeks for an appointment for mine from British embassy

You cannot apply a Non-O (Thai Wife) without a marriage certificate.

You will need a letter from your embassy confirming you are free to marry

You have 90 days to enter Thailand from the time the visa is issued. But as you are not yet married you will have to apply for that Non-O visa for some other reason. If you are over 50 you can apply based on your age, but your 1st 12 month extension of stay normally has to be for the reason the visa was issued, so retirement in your case. So when you get here your priority should be opening a bank account and making sure you have a minimum of 800k baht in it for 2 months (not 60 days) before applying for the extension, it must stay there for 3 months after getting the extension and not drop below 400k baht at any point in the year. On the bright side, it leaves you plenty of time to get married, some immigration offices won't give an extension based on marriage until you've been married for 2 months, and you can swap to an extension based on marriage when you apply for the next extension of stay if you want