I think one of the things you're driving at in your last few posts is, Ryl might not be able to stick with monogamy forever, and you in turn might not be able to tolerate one more bit of polyamory. This combination of opposing forces would, logically speaking, spell the end of your marriage. Or to condense that idea, you're wondering if you and Ryl are really compatible as a married couple after all. Sort of like saying what happens if you take one person who's 100% polyamorous (by orientation), another person who's 100% monogamous (by orientation), and pool the two people into a marriage. Isn't the marriage guaranteed to fail? It seems more like a question of when than if, and the how is just details.
I can see your reasons for not wanting to lead Ryl down that rabbit hole when she's already chafing with regrets, but in part of my mind I wonder if it wouldn't be a good idea to somehow at least let her know that all is perhaps not as well in paradise as she perceives, believes, and thinks. If for no other reason, so that she wasn't blindsided when this "inevitable divorce" materialized. For instance, you might consider pointing out to her that you and she are enjoying something of a "second honeymoon," and you're worried about what happens when the honeymoon's over. I don't think that statement would aggravate her guilt feelings too much, it's just something she could hear as a statement of your (understandable) feelings and perspective.
Perhaps the hitch is if she feels "guilty for being polyamorous." Such guilt feelings could be skewing her naturally inclined thoughts and actions. But in a way you're both in the same boat if you, in turn, have been made to feel "guilty for being monogamous." If the marriage is eventually going to be tested anyhow, would it be wrong to test it somewhat now with admissions of the doubts and fears that currently exist? I don't have the answer to that question, but I reckoned it might be worth mulling over. The potential benefit might be that you and she could both put some of those old guilt trips behind you, even if it meant you had to admit you might not be meant to stay together forever.
I think it's okay to not know whether you're "really compatible" until the passing of time makes that known. Sometimes it's enough to be "in the now," appreciate whatever blessings you have while you have them, and "give them a try;" find out if they'll stick by putting whatever heart and work you can into them. Given Ryl's considerable smarts, I wonder if she wouldn't be able to understand and assimilate all of that. Maybe she needs the chance to hear "the rest of the story" and cope with it in her own time and way. After all, you and she both seem to me to be strong people who can confront the hard truths with objectivity and courage. Some things are hard to hear, but you can still learn to make peace with them over time.
Perhaps the most important thing to remember, no matter how this all plays out in the end, is that you do love each other. If in time you find that you have to part ways, try to do so amicably if at all possible. Try not to let the kids become a wedge that drives you apart. Instead, I'd like to see the kids become a kind of glue that helps you and Ryl stick together at least as friends. Don't blame yourselves or each other for things that "weren't meant to be;" retain the respect and regard you have for each other. Let the kids continue to be involved with both of their parents, and make their own decisions in life as to whether they choose a monogamous or polyamorous path. I state this as if it were meant as advice, but I feel that it is more an expression of my own hopes for you, Ryl, and the kids.
Of course we all hope that things will turn out just fine, that you and Ryl will somehow find your compatibility when the "real work of the relationship begins." Don't undercut yourselves though, I think you've both already been working quite hard on your relationship, even if the "honeymoon aspect" has made the work relatively joyful so far. It's not like the two of you haven't also waded through your fair share of sorrow in the past six months or so.
Maybe the best direction for therapy to take is figuring out whether you and Ryl need to be on the same or separate paths, and to research how to navigate which of the two futures is best without hurting each other (or the kids). I don't say rush into any of these decisions or "expressions of advice," but I offer them up as food for thought for now. Is there any way your therapist might see the benefit in such a shift in direction? Can you share with Ryl some of the deeper fears you've shared here? You do stand to gain some potential closeness, even if it ironically be on those two different paths.
I am sorry about the permanent wounds the past has branded you with. I guess that is one of the mysteries of life, that we all seem to end up with damage we never "really" recover from. Even after all the forgiveness and acceptance we can muster, the past still leaves us with a "twitch." What you can't erase, I guess you just try to learn to live with as best you can. Thus and so are my wistful musings about it all, at any rate.
Love means never having to say, "Put down that meat cleaver!"