Having had a fair bit of experience with the swinging scene in my past, I echo what others are saying about the fact you’re looking for a unicorn. I know you’re talking to someone, there’s a lot of talk talked on swinging sites but it rarely manifests. If you did find someone, you’d have to be offering her ideal situation, otherwise she’ll keep looking. And her ideal situation probably means you being sure it’s what you want to do, which I don’t think you are.
Maybe an easier way to find out would be to go to a swingers club and talk to people, it may or may not lead to any action so to speak, but just being there will probably answer a lot of your questions, in a no pressure environment.
WRT to the question in your OP, I know that I could never share a partner so it’s not something I’ve done, but I have been the OW in a ffm and it was weird. I was a lot younger, they were a married couple. It was their first time and you could tell she was getting really uncomfortable about him touching me, so told him to stop, which was a shite deal for me, so don’t be that person 😂.
What I have far more experience of is being in a relationship where we had MMF threesomes, on a regular basis. My ex was totally into that. We did it for years but eventually it’s what broke us up. One of the men I slept with (a few times), I just had the most amazing sexual chemistry with him and although I knew I didn’t want to be with him, I realised that sexual chemistry was completely lacking with my actual partner, and I ended things. So that’s a danger, there is a possibility your partner may have a similar awakening!