Romance: Allow is the culprit on the divorce. When a household whose relationships you do not want to progress forwards or backwards in your absence is inactive, Romance should be set on Disallow. Unless you want these things to happen and for them to find new RIs, etc. on their own. Romance:Disallow doesn't stop them from being affectionate or from woohooing with each other, it only blocks partnership progression in either direction.
Breakup: Disallow only covers certain breakup scenarios, the ones that SP itself is allowed to push or not. Sometimes sims' relationships just kind of dissolve without a formal breakup routine running if one is caught/accused of cheating and/or they don't like each other very much anymore (probably for a few other reasons). Or the game itself can break them up. The relationship Liking Gate can be adjusted within the mods, and guess there could be exceptions but I usually do not wish to have sims who become bitter enemies with relationship scores of -90 or something stay partnered.
I've seen all combinations of things happen and not happen with those Faithful moodlets. There might be some point to most of it that many of us haven't been able to figure out yet, such as having or keeping the moodlets being related to traits and not affected by cheating if such activity is not discovered by other sims. But we have to believe that some of what makes those moodlets appear or not just doesn't always work in a logical way. I once had a sim with 25 RIs and 28 illegitimate children (not all in the same world) pick up one of those moodlets towards the one sim he was supposedly dating at the time and thought it was funny so I let him keep it for a while.