Welcome first of all. I am so sorry that you had to suffer such a loss.
And secondly: What the ...?! How long did he know her? She really got some nerves inviting herself over and right into your bed when you don't really know her and never heard or thought about something like swinging before. At least as long as you are in the picture, that is what this was about, polyamory is about love, not jumping into bed with strangers.
And thirdly: Again, she got some nerves there. How on earth did she get the idea to go to counceling with you two? That's just so rude and uncalled for. You are absolutely right, it's the loss of your husband and yours to mourn, not her place to get involved at all.
To me, she is invading your relationship and privacy on every possible level, totally disregarding you and your needs. But what is really worrysome here is the behavior of your husband in my opinion. How can he act like that? Maybe this is the aftermath of the loss he experienced or whatever brought him to think that it would be OK for him to do something like this in such a situation. His reaction and demands seem to be kind of extreme.
Have you been able to talk to him about his reasons he wants to pursue this woman and polyamory? And why he can't give you at least some time to think about the concept theoretically? It's highly unreasonable of him to expect you to be OK with something major like this over night after one evening he and a stranger talked to you about it.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.