It should be a choice as to whether you send supplies by sea or pause / slow your offensive while the land supply route gets sorted out.
This in theory can need be fixed like "all puppet gain military access to other puppets under the same masters"
This is a quite different issue to do with issuing movement orders. When instructing units to move to a province by clicking on the province they will ALWAYS take a land route if one is available and only use the naval route if they cannot go by land. If instead you issue orders to be in the destination area (fallback line or frontline or something like that) then the game is more sensible. If you want to give explicit orders then you order them to the port where you want to embark and then order them to the port you want them to land at by clicking on the port symbol (rather than the province and then order to final destination. This sequence can be chained together with shift clicking. The key bit to realise is the need to actually click on the port symbol as the travel destination to specifically prompt naval movement.
The biggest problem is that the rails to St. Petersburg are level 3, whereas the rails to Vladivostok are 2, perhaps some of it is level 1, if I remember. Upgrade the rails to Vladivostok to at least 2, all the way, maybe 3, and that will go away.
I think they would. I always tend to send my puppets some stuff to pad out their inventories though so that's usually less of an issue for me personally. In an instance where supply is travelling from one nation to another though...I'm honestly not sure what the "rules" are specifially.
