Hi prettyinpink,
Some great advice so far. I'll add my perspective in the hope that it helps.
So your husband is also your Dom/Daddy. He has been looking for a slave for 2 years, and has finally found one. Perhaps a little bit of over-enthusiastic NRE-ness going on because of that. Has he ever had other partners before?
Firstly, I can understand why you are upset about him stroking her hair while stroking yours. It must have been very hard for you to share your intimate D/s space with this new person, and I can see why you may have felt a sense of discomfort, violation, or intrusion. While some people enjoy this, or don't mind it, I generally dislike this too, for various reasons.
Before meeting, you expressed that you'd prefer him not to Dom her at the event, and he agreed under protest. Plenty of people prefer to meet new metamours in a non-sexual setting. This is not unusual. It's a shame that your husband couldn't accept that without throwing what sounds like a mini tantrum about wanting to play with her. Some people are self-oriented that way. At the same time, he is allowed to express his desires and wishes just as you are.
Once at the event, you felt disconnected from him because his slave was at his side a lot. Your husband's new slave left for a moment, and he asked you to kneel beside him. She returned and knelt too, which you perhaps felt was unthoughtful, since you'd tried to give the two of them space to connect previously. It sounds like there may have been a bit of rivalry going on, whether just from you, or both of you. This can happen in any poly dynamic, and certainly between submissives

You both want to feel safe, you both want to feel special. There's absolutely nothing wrong with that. I'd try to keep an open mind about her for now, and remember that she would have been wanting to please him as a slave.
You felt that your husband went against his agreement that he would not engage in D/s play with his new slave during that evening and this upset you. I find myself wondering something about this. Perhaps Domming has a different meaning for your husband than it does to you. I can see how kneeling and wearing a collar might be thought of (by some) as a general interaction between a Master and slave, or Dom and submissive. For instance, my partner hasn't yet played with her sub in front of me, but she has treated him submissively (tugging his hair, giving him a light spanking, etc.) These things are uncomfortable for me, and quite profound for her sub, but my GF finds them small and lighthearted. Perhaps your husband thought that you meant "Please don't throw her on the cross and do a 45 minute scene with her"?
You must be able to talk to your husband about this. If you keep it inside, you're not only doing yourself no favours, you're also not allowing your husband the opportunity to grow and reflect. As GalaGirl said, think about how you generally approach your husband. I used to think that I approached my girlfriend gently and logically. Perhaps I did, perhaps I didn't. Either way, it often ended up in a fight. Eventually, two things happened - firstly, I expressed that I'd had enough with being screamed at. It could happen no more. She expressed the same to me. Secondly, I read and read and read about improving my non-violent communication skills. If you haven't read it, I'd strongly recommend reading 'Nonviolent Communication' by Marshall Rosenberg. It has some wonderful pointers.
When talking to him, I'd focus on the reflections you've made. You could tell him that you've realised that meeting a new metamour at a play party isn't comfortable for you, and that sharing D/s moments with someone else (i.e. the hair stroking) is upsetting and invasive for you. You could look inside yourself and see if you can relate to his reasoning before he has the chance to feel defensive. For instance, you could say something like this:
"Honey, I really understand that you were probably trying very hard to make sure both Slave and I felt wanted and comfortable the other night. I know you've been looking for a long time, and it must have been great for you to have two partners with you. But I have to express something, and I hope you'll understand. I found myself very uncomfortable when you touched us both at the same time. I'm nervous about sharing this with you, and I'd love it if we could talk it through."
Additionally, I'm always of the mind that it's better to express feelings and needs rather than ask a partner to provide a fix. For instance, you could have originally said "I'm worried that seeing you interact with Slave in a D/s setting the first time I meet her is going to stir up unnecessary insecurities in me. If you really want her to attend this party and you really want to play with her, it might be better if I skipped this one." That's all about you, your feelings, and you managing your own emotions and situations. Your husband can then say "Ok, great!", or he could have offered something if he'd wanted to.
Sometimes I'll say things like this to my partner. I believe it's important to give a head's up. I might say "Baby, I'm really anxious about meeting New Guy again. I know I've met him before, but I'm finding this tough. You don't have to change anything or do anything differently, but just know that I might need to go out for a smoke a lot, or I might seem quiet." My girlfriend will then be aware of my feelings without feeling obligated to do anything. When I put it like this, however, she's more likely to actively *want* to be considerate. If I say "Please can you take it easy on the touching in front of me?", she feels burdened, uneasy, claustrophobic, pressured, and whatever else.
Finally, you say that your husband invited his slave to the play party. Was this with your consent? Or did he invite her along without asking you if you were happy for him to do so?