Setting my SetMat to Reliable actually made it work as intendet!
As you can see, now all my Meshes are being changed from the Server and then from the Client.
However it still seems off, as I would expect it to have each Material to be Changed on the Server and the Client taking turns like this: (Awesome Paint skills behold!)
Do you have any Idea why the calling this Multicast in a loop would result in this outcome?
Your suggestion still solved my problem of the client not changing the materials, thank you!