sadbodyblue, there's a difference between a healthy dose of cynicism, and a Hercules C-130 loaded with it. It's perfectly normal to say "Some people are going to cheat regardless, and some may cheat due to relationship issues", but saying "I never trust anyone 100% not to cheat" is just ridiculous. Your husband must be a saint to put up with that sort of suspicion constantly.
I do not, in any sense, think the texts sent by Miss Donut are flirty. If she's got his phone number, she'll have texted on behalf of the team to say "We miss you, get well soon!". And too bloody right they should say that! Not only is he their management, if he's their point of contact and organisation, he'll be spending a lot of time around those people, and most likely be their friend.
In the OP's situation, if I had a DP who got those sorts of messages from a work colleague, I'd think "Damn, even though he's their boss, they're still lovely to him. I picked a good'un", followed by lots of (as long as DP wasn't contagious) smooching and a smile.
If there's a back-story, we haven't been given it, and it's naive to assume there is one - if OP wanted support for her cause so badly, and there was a back-story that would help with that, by God, she'd have given it.
"Am I being unreasonable?" - As it stands, yes. Yes, you are. If you've got actual tangible proof to change our minds, by all means, go for it. Until then, this sort of suspicion is just going to drive a wedge between you and your DH, and if you can't talk about it with him, then you're going to make an ex-DP out of him soon enough.
Now, I'm going back to my lazy Sunday afternoon of fancy beers and Doctor Who. Anyone who wants to join me is welcome to.