Can I extend my Non-O spouse visa if my online appointment is two weeks before expiration?

Too late would be May 16th

I think you are misunderstanding the "window of opportunity" you have to apply for a new extension.

At MOST offices you can apply for a new extension when you have 30 days or less left on your stamp <- that means any time from a month before the extension expires right down to the very day it expires.

As long as you're applying before the current extension expires (which you say is May 15th) you're fine.