I wouldn’t take Hollywood movies as a model of anything. Or fiction in general. Schizophrenia (which can involve auditory hallucinations) is different from what was called MPD or DID.
MPD or DID is the idea of a disorder where one person has two or more "personality states" which are reflected in their actions and behavior. In the most pure conception, this would be multiple "identities," each with distinct memories. However, at least from what I've read, it's a very controversial topic and there's a fairly prominent theory within psychiatry that the disorder is actually induced by certain therapeutic techniques or operates via the patients playing a role they believe appropriate for a person with DID.
I agree that it's not going to be over marriage in the near future. In addition to the potential contractual issues, leftists are going to want to avoid association with historical and still practiced
polygamy (even though it'd basically result in this). But I don't really think "marriage" is the defining feature tying the family of issues were talking about together. After all, I'd say that the earlier "LGBT" issue was the legalization vs criminalization of sodomy, which isn't a marriage issue at all.
On incest, I don't really see first cousin incestuous marriage as having the same cultural hot point potential as sodomy marriage. It's something that existed in (and was if anything, more common) in the recent past, and there's not really any cultural heat over the issue at present, at least as far as I can tell, and the states that allow it don't really match up with party lines. I think it might be a good idea for the right to push for total delegalization for immigration patriotism reasons, but I don't really think the left would fight that all that hard.
Close-kin incest and incestuous marriage could be a cultural hot point, but I don't see any sort of widespread or common endorsement of that on the left. I do think that of examples in slippery slope arguments (pedophilia, incest, bestiality, necrophilia, etc.), if we reach that point, incest would be the first, as it can be argued along the "consenting adults" line, and I've seen it be an occasional subject for hot takes among the left along those lines.
The law you're thinking of that was changed wasn't explicitly and openly legalizing pedophilia. IIRC it was changing the law to reduce the least serious offense that a male pedophile who assaulted a boy could be charged with to the least serious offense that a male pedophile who assaulted a girl could be charged with.
I don't think pedophilia is on the horizon for the left. To reiterate, a rule of thumb, think about what you could or couldn't see people supporting on our evil mirror website.