I would say that GFCI just filters current. So Line from panel to the load to the shared neutral. What happ when power travels from opposite side. Unbalanced load flows only behind GFCI. If in CEC doesn't say it clearly, well you can and can't. That right inspectors reserve for themselfs.
If you don't see something a lot in practice people are sceptical about it. GFCI should be checked every month. Not for use everywhere.
In parrallel circuits:
voltage=constant
current=variable
If you split neutral before comming to GFCI you just changed current...
...or I am just talking nonsense
