@ SchroedingersCat: While it’s good that you have a forum to explore your feelings and It’s a bit of a puzzle to me why you’re seeking advice on a polyamory site.
You’re not polyamorous and it’s clear that you husband’s not either, nor are you down for it if he were.
By your statements you’re suspicious of his lying to you by omission and you’re suspicious of her lying to you to “tell you what you want to hear”.
In the beginning implied that you’re okay with his having your friend for a FWB as long as she doesn’t love him, yet you’re afraid of what will happen if they have sex.
How this is different from simple jealousy in a monogamous marriage?
While you’re deserving of all the respect, generosity and empathy that any human being is, and it’s good that you can talk to people on here about your feelings it may be more effective to deal with your feelings another way.
Now I expect you have may some angry things to say to me, so let fly.