To answer your last question - yes you can reroute continents. I once had a ticket issued CAI-xLHR-Asia-NAm-Eur-CAI and when I landed in London my TA had found me an A seat on the Concorde flight going that day (which was my last chance to fly on it) so I did a reroute at LHR T4 to go CAI-xLHR-xNAm-Asia-Eur-CAI.

BA transit ticketing desk took a couple of hours to do it, but I had time and they let me go through to the Concorde room to wait while they did the reissue.