I don't have the experience you do, but this is what I think:

As a man, in some sense, you are forever alone. You are expected to be a pillar for others, and you can take pride in being one, but you won't get your own one -- especially not from you female partner, when another attractive male is around. Because this is how human attraction is wired: as a man, independence makes you attractive, dependence makes you unattractive. Situations like the one you described create a vicious circle, because the more you need support, the less likely you are to get it, precisely because needing support makes you less attractive.

(To avoid absolute terms: It is a sign of a good relationship that you can get moderate amounts of support. But that's only on a good day, and in a limited amount. Don't try this on a wrong day. And don't get used to it... because sooner or later the wrong day will come and you may not notice it.)

In some sense, men are competing all their lives. On a lucky day, we are allowed to run less quickly. But it's always about what value we can provide. Being sad or begging - that's no value. It's not about what you need; it's about what you can give... or threaten to withhold. The proper expression of jealousy is one that makes the partner think "I am getting some value from this relationship, and I don't want to risk losing it". If the partner merely thinks "oh, I see he is suffering... however, I have a duty to myself to make myself happy", such jealousy only hurts your position.

show signs of love, fondness, affection

To put it bluntly, here you have mistaken your needs for her needs. It was you who wanted to be loved, to be treated with affection. It is a very natural mistake; many people do it instinctively.

What you probably should have done is to find some interesting things to do, and to do them without her. To report feeling great and having a good time. With some hints that other women are interested in you (but you, of course, prefer her). This is what would make her think that maybe she could lose something. -- I realize that this requires a lot of mental strength, and is probably impossible in a given situation; especially if you wait too long.

The worst part is that if you look at it from the punishment/reward viewpoint, you were rewarding her for the actions that hurt you. She received the attention from the new guy and the attention from you. And I'd bet you also did most of the work at home. You contributed to her having a perfect honeymoon... with the new guy. (This is just my guess, but it's based on my model that you tried hard to do everything to make her happy.) -- Again, a better strategy would be to leave a lot of work for her, so she is somewhat tired and angry when dating the other guy.

I considered polyamory.

You mean the kind where also you have alternative partners, or only the kind where she does. What specific steps did you do to obtain the alternative partners for you? -- In other words, were you trying to find a new fair model for you both, or were you just torturing yourself and experimenting how far can you bend before you break?

I was shown the truth. Told unambiguously that she did love another. That I was powerless and was completely at her mercy.

First part was true, the second part was a crippling lie. You could have learned about how to navigate relationships from a male-friendly source. If the "experienced moderator" did not suggest this, I would consider it an evidence of huge incompetence. To put it bluntly, while you waited for her decision, you could have fucked the proverbial ten other women. And you would feel much better, like a valuable human being. And the ironic part is that afterwards you would have a greater chance of winning her back.

If she still loves me I want to believe that she still loves me. If she doesn't love me any more I want to believe that she doesn't love me any more.

This assumes that whether she loves you is fixed, and your reactions do not influence it (which is a crappy way to treat relationships in general); so all you can do is passively learn the fact and helplessly accept it.

I got along much better with her. I could set boundaries (which I previously couldn't; before I'd rather compromise or retreat). And the boundaries were accepted. A clear "no" worked much better than "I will try my best" before.

PUA 101. Congratulations for being able to discover it on your own. But your life would be better if you knew it sooner.

Congratulations for not moving out. Seem like even the law prefers to side with the stronger guy; or more precisely, has a strong status quo bias. If you showed weakness by moving out, the court would simply bless the status quo, because that would be the easiest thing to do.

Assuming that saving your relationship was not an option, you probably got the best possible result, which is impressive. I hope you'll find more peace, friendship, and a new partner (or multiple partners, if you wish -- you have no obligations now).