To kick off thread #2, here's some relevant reading on adult attachment styles. This excerpt pretty much describes the dynamic between H and I that lead to the breakdown of the R.
Additional studies have investigated chronic stress—especially the transition to parenthood . Having a baby is a joyful but chronically stressful experience, making it ideal to test stress-diathesis processes, particularly if attachment insecurity makes individuals more vulnerable to personal and interpersonal problems . Indeed, highly anxious women enter the transition to parenthood perceiving lower levels of spousal support, which predicts sharper declines in marital satisfaction [41,42] and increases in depressive symptoms [43,44] across the transition. Their husbands show parallel effects, including declines in both marital satisfaction and support-provision over time. Highly avoidant individuals—especially men—who believe their newborn is interfering with their personal or work lives  or who perceive they are doing too much childcare  also report steep declines in marital satisfaction. Less avoidant individuals (who tend to be more secure) report much smaller postnatal changes in satisfaction and depressive symptoms.
In sum, specific situations/events during the transition to parenthood tend to activate or exacerbate the cardinal concerns of highly anxious and highly avoidant people—abandonment/loss for anxious persons, and lack of autonomy/independence for avoidant persons—which in turn have negative effects on their marital satisfaction and depressive symptoms over time.
I believe H's attachment style is dismissive-avoidant.
Avoidant: High on avoidance, low on anxiety. Uncomfortable with closeness and primarily values independence and freedom; not worried about partner’s availability. “I am uncomfortable being close to others. I find it difficult to trust and depend on others and prefer that others do not depend on me. It is very important that I feel independent and self-sufficient. My partner wants me to be more intimate than I am comfortable being.”
Mine might be described as secure with anxious tendencies.
Secure: Low on avoidance, low on anxiety. Comfortable with intimacy; not worried about rejection or preoccupied with the relationship. “It is easy for me to get close to others, and I am comfortable depending on them and having them depend on me. I don’t worry about being abandoned or about someone getting too close to me.”
Anxious: Low on avoidance, high on anxiety. Crave closeness and intimacy, very insecure about the relationship. “I want to be extremely emotionally close (merge) with others, but others are reluctant to get as close as I would like. I often worry that my partner doesn’t love or value me and will abandon me. My inordinate need for closeness scares people away.
It's possible that we brought out the worst in each other and that this was always doomed to fail; it just needed a trigger point.
W32 X30 S3
June 2019 | Runaway husband May 2020 | Legal separation Dec 2020 | Happily divorced