expression runs on the server only (just like all other Eden expression) and after other Entity Attributes.
Arma seems to foricbly add quotes around the expression if it has no quotes:

If the fix with resolving them on mission start also fixes the copy/paste and composition related limitations, then yes that should be all.

I don't know if this is the right place, but the custom connections should also be saved by the editor. Right now it will allow you to add the connection but they will be removed on reloading the mission in the editor.

As someone who is currently trying to script custom connections, this would be a great feature.