Hi! I've been lurking here, giving advice sometimes. And now I need some of my own. Tried to structure my questions/dilemmas but please feel free to respond how you like :)

Sorry for being so wordy, I'm not sure how much info is relevant...

Questions first, context later:

1. My girlfriend "Sophie" is mostly against the idea of ENM, but I offered her complete sexual freedom to explore, in the hopes of raising her interest/lowering her misgivings. Is that ethical, valid, smart?
   
2. While traveling alone she had sex & fell in love with "Richard". It has impacted our ability to restart our sex life after her return, which I find less than ideal. Also she's still not on board with the ENM thing. She'll be traveling again tomorrow for more than a week, where she'll see him again. Should I revoke her sexual freedom or let it play out?
3. Sophie told me that he's good in bed, and he's really well endowed. Part of my ENM preference has to do with a stag/vixen or cuckold kink, that Sophie knows a little of in general terms, but hasn't yet shown an interest in exploring with me... Should I disclose now or later, or at all, that, in addition to all the other feels, it turns me on to know she's getting really good sex?
4. I'm looking for ideas on how to move forward after Sophie gets back. Richard will be leaving our continent by the end of July to go back to the other side of the world, and depending on how things go they might make plans for one more get-together. I'm aware that the outcome of the whole episode might still be that we're not opening up the relationship in any way, but I'm still hoping she'll change her mind. Any suggestions are welcome!
5. Sophie is experiencing her affair quite intensely... a bit like a first time XTC in a good way (which I felt was overwhelming because the euphoria was so unexpected). Is this what NRE is? I love for her to feel that, but I'm not keen on how it negatively impacts her sexual interest in me. Question for those with more ENM experience: do people typically get better in navigating NRE and primary relationship? What are respectful ways for me to ask for my relationship needs being met?
6. Am I overlooking red flags in her behaviour or my own because I want ENM too much? (I'm willing to overlook or work past a lot of mistakes, but this is uncharted territory for me and I'm not sure if I'm seeing things clearly).

Context
Sophie (F39) and I (M49) have been together for 4.5 years, and while we don't live together we're committed. I've been wanting a non-monogamous relationship, and part of my initial attraction to her was that she already had two other lovers when we first started seeing each other, so it felt as if some form of ENM was on the table. However, the lovers quickly fell by the wayside and during covid she has made it clear she's not interested in ENM or polyamory.

Talking about it has proved difficult: She's been quick to dismiss the topic with the statement that she'll never be okay with me having sex with someone else, and is just not that interested in the subject. Also she felt I'm crossing her boundaries by 'sexualising her sexuality'. Which is a fair point I guess: I would love to explore a hotwife/cuckold/voyeurism kink with her as part of an ENM relationship.

Unwilling to close the topic altogether, I have on multiple occasions told her that as far as I'm concerned she has blanket freedom to explore sexually if her need arises, without expectations of reciprocity.

While traveling
Over a month ago while on a walking pilgrimage she took me up on my offer: she had sex with Richard, a trail buddy. They had sex again the next day, and it quickly spiralled into a traveling love affair. The following three weeks they spent most of their time together.

While I knew or even hoped something might happen, I was unprepared for the depth of connection she established with her lover, and it feels like we skipped not a few steps, but most of the journey towards a open/poly relationship. This didn't dawn on me right away, as communication from her side was sparse.

After she texted me about the first time they had sex, we discussed over the phone how we both felt about it and I let her know I was generally okay with things while feeling all the feels... from excitement and compersion to jealous insecurity and back. She presented the first 2 times as a YOLO type of situation, and she told me he has a really big dick, which plays right into my kinks! After the second time she told me she would be okay with me kissing someone else if I felt so inclined, but not to have sex, and not with anyone she knows or in circles she frequents.
The following days she more or less stopped sharing info with me, and was mostly unresponsive to my messages, even though I had let her know I would love some extra affirmations from her end going forward. I felt let down and when I communicated this to her a few days later she responded by breaking off the fling, because she felt unable to divide her attention between Richard and me. They would continue as platonic friends. I responded that she needn't have broken up with him on my account, but just wanted to be reassured we'd still be okay.

A few days later Sophie asked how I'd feel about her and Richard renting a car together to go off trail to explore a different part of the region. I told her I'd be fine with it, and I'd also be fine if they decided to have sex again. She didn't disabuse me of the notion that they were continuing as a platonic friendship, and dodged my questions (perhaps lied) about picking up the sexual nature of their relationship for nearly 2 weeks until her return. Just before she got back she stated would probably need some time to get used to me again, and finally admitted to getting re-involved romantically and sexually.

after traveling
Sophie's been home for a month. During that time we've only had sex once, last week, after I told her how disappointed I was that she seemed not sexually interested in me at all... Our sex life was not in a great place before she left, and just before she decided to go traveling I had expressed I wanted us to start working on rekindling and improving our eroticism. The next day she told me she had felt pressured to have sex with me, which made me feel rejected and extremely shameful.

While I realise our sex life had gotten stale before she went traveling, and while I'm okay with her having sex with someone else, I can't help but ascribe our lack or sexual reconnection with her obvious feelings for Richard. We're quite close otherwise and I'm willing to be patient to allow Sophie to reconnect with me on her own terms/time, but I've been feeling quite insecure about it. What complicates things is that tomorrow she'll be leaving again for 9 days, during which she'll spend more time with Richard. They're both traveling to a city outside of my country where they'll stay in the same apartment.

While talking about this a few days ago she told me I could withdraw my permission if I wanted. After thinking this over I told her I wouldn't engage with her offer, because my conviction in ENM is that we have a right and responsibility for our own feelings and actions: she's either mono or ENM, and it's ultimately up to her how much she wants to open up her heart, mind, and body to me and/or Richard. My concern is how she feels about me, and limiting her behaviour towards him changes nothing for me. She found this a very odd way of looking at things, and doesn't agree with it. In her mind, relationship partners should be able to withdraw permission whenever they feel the need.

going forward
We both view the next week with some trepidation. Yesterday Sophie confessed she's having mixed feelings about seeing Richard again. She was even contemplating calling the whole thing off but also felt that would be unkind to him, and I agreed. I for my part am in an emotional vortex: (sexually) frustrated/chagrined*/kinky-excited/shameful** for her to continue her affair/worried about our future together/eager for the opportunities this might open up for us...
The fact that I'm currently super into Sophie in part because there's another lover on the scene hasn't escaped her. What remains to be seen is if she can work with that, or is repulsed by it, or something else entirely. If our relationship survives the next couple of weeks, I hope to finally start exploring our sexuality more fully together! Think tantra workshop, going to a sex party, inviting a third into our bed, swinging, and dating separately if she's up for it. I would be open to us evolving to a poly couple but I'd already be over the moon if we get even halfway there!

Even though this episode is costing both me (and Sophie!) a lot of emotional energy, it's a very educational time, and exciting in ways I've never realised were possible. So even if this all ends badly or fizzles out; part of me will always be grateful for the feelings and insights I'm gaining into myself, as well as in her.

If you made it all the way here: thank you for your time! :)

* chagrined, because between Sophie, Richard, and me, I'm the only one advocating ENM while also being the only one not actively benefitting of this situation.
**shameful, because we're sort of unwittingly acting out a cuckold scenario that none of us has actually agreed to.