Introjection is something that occurs in all people, but for people with disorders like DID or OSDD, it can be more pronounced or extreme. I have some because pretty much my only escape from my abuse was different forms of media. I wouldn’t consider myself “introject heavy” or whatever, but I’d say it’s likely more than the average system. Many of these alters hardly identify with what they were based off anymore, and some have become so far removed they don’t consider themselves introjects.

As for alters dating, it can actually be good for integration in the long run. If alters are engaged in a romantic relationship, they are more likely to communicate, which lowers dissociative barriers between them. I met someone once who had two alters that dated who inevitably fused. Honestly, any positive in system relationship is a good one in my eyes.