Men and women can be platonic friends. Best friends.
But they don't behave the way your husband is behaving. The secretiveness, the disregard and lack of respect for you and your feelings... I don't think you are being unreasonable at all.
I had an emotional affair with someone at work once, decades ago. We never crossed any physical line, and I didn't really want to - I didn't particularly fancy him. But I did love him, very very deeply, and I did want to be with him all the time, and the feelings I had for him were more intense than those I had for my boyfriend at the time.
Looking back, it was strange - a rabbithole I fell down so easily, but by stealth. I didn't fancy him. There was no physical attraction on my part. But by the end, I was in deep. It was very confusing.
So giving your husband the benefit of the doubt, he could be as confused as I was. That doesn't change a thing for you though - he's treating you horribly badly. My 'work husband' definitely got the best of me at that time, I thought of him before my boyfriend, I couldn't get enough of him, we went out for lunch every day, he drove me home even though it was miles out of his way, we had a Brangelina-style name bestowed upon us by people at work because it was never one of us, it was always both of us, something we leaned into by signing birthday cards and leaving cards together, like a couple. I know. Pathetic.
If my boyfriend had asked if we were having an affair, I'd have truthfully said no. If he'd asked whether I wanted to, I could also truthfully have said no. I don't think he would have thought to ask the right questions - do you love him as more than a friend? Is he more important to you than me right now?
It's not a part of my life I am proud of, on many levels. My boyfriend deserved so much better, but so did my EA. It ended because he was painfully in love with me and couldn't stand being in my orbit any longer, as friends - so he cut me off dead and moved jobs. It was an absolute fucking mess, and one you're potentially on the edge of.
If she's that good a friend, she should be part of your life too. My husband messages a few of his friends a fair bit. I know them all, we hang out together on occassion. So invite her over for food. Insist on seeing the messages they exchange. If he's twitchy about any of it, it's not you. It's him.