I dunno if you have this kind of support, but for me feeling out boundries was easier because I knew I had Karma's support. When we discussed it I told him "I am trying to learn to be okay with this and to deal with all these emotions. I am honestly trying. I need your support and your patience. I may give you the green light for something and have to retract it later. I need you to know this is not out of malice, it is out of my own needs. I need to know I have your support in that. I know you won't be happy and I will try my best to not yo-yo you, but I need you to understand that this is new and raw to me and I'm not going to know what I am okay with and not okay with until I step off the ledge and see what happens." And Karma was loving and supportive, frustrated, but supportive.
You can't promise not to upset them, did they promise not to upset you when they had the affair? No. From my experience I can say there will be times when any or all of you will be upset over something in the dynamic. It's gonna happen because it is so new and raw.
It sounds like you have a lot of healing still to do. I encourage you to stop worrying so much about them and their dynamic and your dynamic with them and look more inward. Take care of you and what you need to heal. They may not be happy with holding on while you find what you need, but ya know what, you didn't ask to have your world rocked and you're working on healing a lot of wounds and they need to be patient with that.
Boundries for me were baby steps.
The first time they saw eachother after I found out it was here in the living room with me in the bedroom. I couldn't see her, see them. It was too much. I spent those few hours crying into my journal. But I wasn't going to go forward by standing still. So my first boundry, rule, step, whatever was you can see her here only. I go with you to pick her up and drop her off. That lasted a few weeks. Then I would let him pick her up and drop her off without me but visits were still here and only with me home. Slowly we went to parties together. More for me to adjust than anything else. To put a social face out there so no one felt need to pitty me.
As we moved forward I became more okay with them being alone together.
Things ended about 7 months after I found out and by that time I was okay with everything other than sleepovers at her place. I need him home at night.
They didn't like a lot of the boundries and restrictions. They didn't like having to sleep on the couch, they didn't like waiting for me to give the go ahead for them to start sleeping together again, they didn't like having to relay every detail of their time together. But that was what I needed to regain trust. Right wrong whatever. It's what I needed and I'll not apologize for it. I made him give me his e-mail passwords and his voicemail password. He was not allowed to delete anything. Any phone conversation happened in front of me. Karma had NO privacy for a long time.
And now, I could careless. I trust him. I know why he was afraid to be honest with me and we've worked on it and moved forward.
I still have things that will trigger the trust issues, but they are few.
I can't tell you what boundries you will need. I know for me at first the thought of them holding hands hit me like a brick. the thought of them kissing, expressing any form of emotion towards eachother just ripped me apart. But I realized, mostly through reading on here, that this was my problem not theirs. I had to work on me to be okay with it. Other than respecting my boundries and working to regain that trust, there was nothing they could do to make me heal. I had to make me heal.
I found a lot of confidence in myself in this time. I relearned how to be self aware and self relient. As painful as it was, this was one of the best things to ever happen to me because I am now so much more aware of myself and my needs. I am so much more confident in who I am. I have a lot more respect for myself now.
It is hard, painful work, but you can move through it. Sometimes it may seem like you are standing still. Karma and I had a few fights because to him I wasn't making any progress, but I was, it was internal, he couldn't see it, so he got frustrated, but I knew I was making progress. Which is way I say don't hide your emotion. I hid a lot of the hurt at first and to me, as I slowly became more okay with things, that was progress, but since Karma didn't know where I started, he didn't know how far I had come.