>>Will armed pilots only operate on domestic US sectors? Can't imagine how they would want to go to the hassle and expense of complying with - say - UK firearms legislation were they to land in LHR. As soon as the doors open they would become a criminal...<<
Don't know how familiar you are with UK firearms laws but several air carriers operate into LHR with weapons and comply with local regulations. El Al is the most visible but there are others...
I'm not crazy about the guns myself but I have operated into LHR with armed crewmembers.