What is in the inactive route will not be shared via ACARS, regardless of what operator options are chosen.
Bear with me for a moment - I'm trying to fit together bits from various posts and get my head around the whole thing. Is the following correct?
The "B" route is not shared via ACARS until it is selected. When/if it is selected, it shares essentially a step ahead of what the plane is currently doing? So if you programmed the plane to fly straight for ten minutes (or miles, or to a specific point) and then turn to a specific heading, ACARS would send out the information that you intended to turn while you were still going straight?
If I'm correct on this - that could explain a lot. PF pre-programmed route B with the hijack route - but the first 10 minutes of that route were the same as route A. While the PNF is distracted, PF activates route B. Not knowing of the change, PNF proceeds to contact ATC. PF then gets PNF out of the cockpit, locks the door, and flight turns and continues along the remainder of route B.