The collections would be created by the server with no user/external input, it s

The collections would be created by the server with no user/external input, it should just exist - the way outboxes "exist"

I suggest adding this requirement to the FEP. Some server implementations don't contain application-specific logic, and only support explicit creation of collections.

Using standard AP C2S?

Not necessarily. Any kind of C2S that results in activities being added to outbox.

Create for new emojis, Add to append, Remove to remove, Delete to delete an emoji entirely.
This is already specified in the AP spec.

I think this could also be clarified in the FEP. For example, an alternative to this is Update(Collection) activity.

Create for new emojis, Add to append, Remove to remove, Delete to delete an emoji entirely.

What should a client put in their to and cc fields?