The problem is, that even though you say you've resolved all your relationship issues, in truth you haven't. You wouldn't be fretting over the fact your partner/husband has unblocked a colleague who he had a brief fling with and cheated on you. Ultimately you don't trust him. He's lied to you in the past, and has the habit of telling silly lies to try and worm is way out of perceived trouble. That behaviour in itself is immature, it's what children do.
I can't say why your partner/husband has unblocked this woman. Instead of being honest, he's lied...again! He can't really give you a plausible explanation, because there isn't one!! I suspect it's to see if she contacts him, because then he can say he hasn't contacted her, it was all on her - which to a degree is correct. He may not have contacted her, but he's given her a way to contact him, if she so wishes!!
This is what happens when we 'forgive' someone for cheating and even work through the relationship issues. The trust is usually broken, and when something small like you are experiencing crops up, it magnifies just how much that trust is broken. Yes, sometimes the trust can be repaired successfully, the cheater is remorseful and never strays again. More often than not, the trust isn't there any more, couples limp along for awhile, until the relationship breaks down again.
End the relationship. It's run it's course. You've tried but If you don't or can't trust your partner, what's the point in continuing?