Thanks for sharing the video.
Regarding the difference in the vertex count we will need to understand where it is coming from. Head asset is actually composed out of multiple meshes such as head, teeth, eyes, eyelashes, etc… And all of these meshes have well defined topology which has to be the same across all the characters. As mentioned, only exception is groom, so things like hair, beards, mustaches but also eyebrows. Maybe this difference is coming from different eyebrows selection on different characters. That would be my guess. Simple fix would be to open these Metahumans in MHC and make sure they have same eyebrows selected. Once that change is done, you could save changes and download it again through the Bridge.
A side note, while making sure that you have same eyebrows on characters, you could also select same body type for all of them. That will align their heights and necks. By doing so, you won’t need to go through the step of head alignment. Necks and heads should be aligned perfectly.
Hope this helps.