Hey for anyone following this! I managed to get things working. Using player model lib now even though it's not technically a dependency (just using for the scrolling feature, swapped off of old scrolling)
Managed to get this!
For anybody ever interested, I ended up just replacing the entirety of the section as seen below. This means all of the OLD mustaches (and in the future hair, beards, hair extras) are just contained within the mod folder now.
Trying to addmerge a shape into an existing path without being able to have the game search in two different locations seems to just not function. Best option is to either create a whole new section (I compared my work to the goblin ears mod https://mods.vintagestory.at/goblinears Thanks @Aillun!) or to replace the entire section with a new folder.
The "Object reference not set to an instance of an object." crash was due to the game being unable to pull the shape file, I'm assuming, which is what I set out to fix.
Now to figure out how to get textures to overlay....
EDIT: Textures weren't mapped correctly. Was using the seraph-hair.png, should have been using lightgray.png... for some reason.