As the wife in a group of friends of such long-standing its tricky. As although you invited her to your 'dinner only' part, its not you and her that are friends, and i think its a blokes stag so his choice whether he'd like that to be a celebration of male time or open it to women also.
As the wife of one of the long-standing friends that does join in, she would be naturally on the outside of any stag/hen do arrangements. As, i imagine, would be all the other OH of the male hobbiers?
I wouldnt ask to go to a stag do, nor would i feel i was entitled to join the hen do of someone i barely knew. Its a judgement call by the marrying couple.
She is part of the hobbying group by merit of the fact she is married to one of the longstanding male friends, is all.
None of this explains to me why she would be off with you OP.
If I were being hosted at anothers i would consider myself very rude to ignore any other members of family when spoken to. In their home, its very ignorant way to go on.
However, she is sometimes pleasant!
If she has been pleasant to you, then i also struggle to see that she has a problem with you? More likely shes just not really taking much notice of you as you are not really part of the evening, or shes struggling with her own issues.
Your DH is weird to be setting himself against you on this and triangulating. Especially the strength of his reaction! There is reason he is so reactive against you. Is he normally like this?
If a female associate of mine blanked my dh id want to be asking what was going on, not that dh would expect that but it is my associate? Id at least want to say, "what are you doing?"
That he doesnt want to do that is odd.
I would have thought that if there were ongoing issues of any sort with this woman he would have said at the outset that she's this way with everyone. So she isnt this way with the male group.
I certain wouldnt want to be 'keeping an eye on her' i find that a weird response.
I think hes very lucky to have you op, to have opened your house every week for 4.5 years to a group (so taken over a large area of your home one night every week, expecting to have the night off parenting duties also).
Its reasonable to share the hosting, of course it is. Once every month or six weeks (however many involved), and twice at hers.
I would ask dh, is this behaviour particular to you or is she this way with everyone.
I wonder whats going on between him and her potentially, because of both their behaviours.