I spent (too) many years dealing with my lying, cheating, (now)stbx. Whenever I heard language like your WH is using with you -- it ALWAYS meant there was *more*. Always. Every.Single.Time.
So, did I read this right? He says it was 2 1/2 - 3 years....but you uncovered evidence that his acquaintance with this OW pre-dated that by 4 years? (making it more like 7 years??!??) That's a pretty big difference in time. Your WH may not remember specific dates....but I'll bet you dollars to donuts that he *remembers* a lot more than he's willing to admit right now. And he is trying to bully to into backing down because he doesn't want to be honest with you......"Oh, what a tangled web we weave when first we practice to deceive." - Sir Walter Scott
In my effort to be *concise*, I often come off as blunt and harsh. Sorry, don't mean to be offensive.