A Dissociate Identity Disorder sufferer discovers his alternate personalities are actually various versions of himself existing in different timelines and must use his connection to them to help them prevent a disturbed alternate from finding and killing the original version of himself, which would erase them all.

You’ll struggle to communicate the basic logic behind this premise in a pitch much less in a synopsis.
If each personality exists simultaneously in a different timeline, then each of them is the original version as far as that timeline is concerned. What do you mean by “…original version…”?

Logic aside, I think the intention is that the disturbed alternate personality is suicidal but the others are not and they have to work together to stop the bad alternate personality from destroying them all. It really is a story of self-preservation, the MC’s main concern is himself, is there any way you can make it so he needs to stop the bad alternate personality for the greater good instead of just himself?

Bear in mind that the premise of a Dissociate Identity Disorder sufferer working with the different personalities to stop one “bad” personality has been done in Split. Maybe the whole parallel timeline aspect will add enough to make this original, but at the same time, it’s worth considering the fact that it won’t be and producers will see this as a rehashing of Split regardless.