tldr; No, that did not fix it.
I checked my other rollback saves, and fortunately had one just before the bugged couple did their vows, they did have the Engaged! moodlet at that time. I used that save to play forward about a day and a half for their engaged buff to expire (and to do some unrelated uni classes for the groom), without getting them married.
Once it expired (and classes were over) I took them back to the same scenic spot in Tartosa where they had eloped in the original save, to now elope without the "Engaged!" moodlet. I did the vows from the main household sim's perspective this time (and fortunately the noncommittal spouse didn't reject it, lol). I did not move them in together, main household is still full (and I didn't the first time). I did invite the spouse to the dynasty. I had them do a couple random social interactions, to give it a minute to make sure the marriage was really complete, walked them back to the active lot, made sure they had the Spouse relationship bit, and entered CAS from a mirror. On exiting CAS, I received a message that they were partners. He had the propose interaction again, and the profile had the Partners relationship bit instead of Spouse.
I saved just before going into CAS, so I exited without saving and reloaded the game. This time I sent the sim home before going to CAS, which had the same result of resetting them to Partners.
I also saved right before the vows in this post-"Engaged!" continuation. So I restored to that save, to try moving the main household sim into the overflow household with his spouse after the vows. Repeated vows, moved them in together, invited to dynasty, returned to active lot, entered CAS through mirror. Same Partner result. Exited without saving.
Next I returned to right before the vows, to try a MWS custom wedding event. (It worked for his sister! Her marriage has been intact through all of this, and I keep checking hers every time too.) I held the event at their home lot, since that's where his sister's wedding event was. She was added to the dynasty after the vows during the event. I did not add the spouse to the household at the end of the event. I proceeded to enter CAS via a mirror after the wedding event ended (naturally, I didn't end it early), and STILL got the same Partner message and relationship bit.
During all this, I also noticed that the spouse was pregnant. In the initial save, she did not have a moodlet for the pregnancy yet at the time of the marriage, but I kind of remember doing a TFB at some point earlier (I don't think I have a save back far enough for before it, especially since I can't remember exactly when it was). However in this recovery by the time of the post-Engaged! moodlet wedding she is visibly very pregnant. My couple that isn't bugged was not pregnant before or at the time of their wedding, however they are now pregnant; Their marriage has still remained intact after becoming pregnant through all the failed weddings of the bugged couple.
Sooo I'm returning to after the Engaged buff ended again, but before trying to get married, and I will play a few more days until she has the baby, and then see if it's because the wedding is during a pregnancy? This will take me a little bit to report back again, so in the meantime, were other peoples sims pregnant when they got married?