I came to poly via starting swinging when I was 19.
Swinging was a lot of fun!
Then I accidentally acquired a girlfriend. Then I accidentally acquired a boyfriend. Then my husband got a girlfriend too
Eventually we decided we were poly, and that swinging wasn't really our thing. But casual sex with people we knew and liked and would socialise with, that WAS our thing. (Although in all honestly we went back to a swinging club last weekend, and we really clicked with a friend, which we weren't expecting at all!)
It might be that Bella-wife wants more attachments than swinging typically allows - maybe the "one and done" approach, as many swingers prefer to do, leaves her feeling like crap. But it sounds like it works very well for Bella-husband.
Moving from swinging to poly is also a little different - suddenly there are dates, where the other person is out with someone new, and you're at home alone. And suddenly you're thinking about how it just feels different when your partner really has emotions involved with her other sexual partners - what makes *your* relationship with him/her special?
I also don't consider swingers to be "not-vanilla". I'm into kink, and I can tell you that vanilla swingers and monogamous kinksters are equally disgusted by each other's existence, and I think the judgement on both sides is ridiculous and pointless.
It sounds like Bella-wife is doing something I tend to do, which is unhelpful. It's not conscious, but basically it's "I feel bad! I don't want to feel bad! I'm a good person! What about this time when you did something that made me feel bad! If I'm not right about being good NOW, I'm still right about you being bad in the past! Now it's your turn to feel bad!"
It is extremely unhelpful.
Don't change the argument.
Bella-wife broke agreements. That's the issue. Whatever her reasons. She could have spoken to Bella-husband about them. What was stopping her? Did she get the feeling he would already say no? Is there already resentment and trust issues? Do the two of you really listen to each other, accept each other's feelings, whether they're based in reality or not?
Bella-husband needs to think about what he wants. He feels betrayed, lied to, and like he has been restricted pointlessly by someone who went behind his back and helped themselves to stuff they are not allowing him.
Bella-couple need to work together if they both want to stay together.
I think moving the artist/cheating partner in with the two of you would be a truly awful idea. He and Bella-wife did nothing wrong, now they get what they want. Bella-husband was lied to (and is being asked to lie), and now he has to live with two liars, one of which he barely even views as a friend anymore?
I've been with my husband for 12 years. I recently ended a relationship which I was in for 8 years. I have some other relationships as well. I have many poly friends. Some relationships went well, some went badly. Same as my monogamous friends, and same as my swinging friends!
Poly works just fine, so long as the people involved tell each other what their feelings are, and agree on boundaries that they can stick to. Compatible communication styles are also pretty important, especially if you don't see each other often. But it can be as simple as one person loathing confrontations and trying to avoid them at all costs, and the other relentlessly pursuing them throughout the house trying to discuss it to death and find a conclusion at any cost.
I've been with my husband for 12 years, and everyone else however long; the only thing I've ever done that was cheating was keeping in email contact with an ex, when my husband wanted me to break ties with him. I hid it because I wanted to do it so badly, it felt natural, and I thought it was a stupid rule. And I thought, oh well, if he doesn't know about it, it won't hurt him.
That wasn't true though. It did hurt him, and it took him a while to trust me afterwards. And hiding it made me feel shitty. And actually talking to the guy made me feel guilty, as despite it not going against the letter of my agreements with my husband, it did go against the spirit of them.
If you think you'll have to keep it a secret because your partner won't like it, it's probably cheating. And you need to either re-negotiate your agreements with your partner, or find partners who do not require you to do things that go against your desires.