Originally Posted by gloopy

In your exact example, you would "flow back" to an open cabin seat if there is one. However once you book the jumpseat its yours, and if you lose out to the FAA or some other higher priority (it won't be a line pilot as once you book it seniority has nothing to do with it) you will get a cabin seat.

We should be able to non-rev list with a move up to the JS if the back is full, thus preserving our cabin priority etc. but so far we can't.

This is a long standing IT issue. You are not supposed to "dual list". You either have to choose one, or talk to the gate agent and if they are awesome (many are) you can say "here's my listing but if I can't get on I'd like to list for the JS" or something like that. You could still get burned by a company pilot actually booking it, but that should at least protect you from an OAL walk up. However, if you really need to go and the JS is available, just book it and you own it.

However sometimes company pilots don't show, so if you're listed in the back because of that you could still get burned by an OAL who nabs the JS at the last second and runs down the jetbridge and gets the JS on a full flight leaving you stranded before you even realize what's going on. We need an IT fix to address this, so for now a good gate agent and/or extreme situational awareness on your part is the only defense.