It seems i was mislead by this Nik's reply (https://igniterealtime.jiveon.com/message/235680#235680 ) which inclined that Carbons will work only for the sent messages. So i have only tested this one way. Now i gave it a try again with Yaxim (on android) and Jitsi 2.5.5305 nightly build (desktop) logged with the same user (on igniterealtime.org Openfire server, which is 3.10.0 alpha i believe) and the Spark on another side with another user. Works both ways indeed. Yaxim and Jitsi received both outgoing and incoming messages. I didn't have to enable anything. That's great. So we don't need to do anything about the route.all. Just need clients to support that
Of course, Google is storing history on the server, so you receive the same conversation even if one resource was offline. That's probably the part about Message Archiving integration in the mix that Nik was talking about. But then you would have to store messages on the server and with large userbase it could take lots of space.