I covered that in my post above.I’m not a coder, so I could be way off, but I don’t see why you couldn’t have longer hand in times.
Yes, you need to do the mission within 6 hours because of the BGS, but why can’t a mission register for the BGS when you’ve completed the task, and then you hand it in at your (relative) leisure?
Extending hand-in times allows a level of exploitability by banking certain mission types/actions.
For example, missions have no influence effect during wartime, so handing them in only helps win the war, but only marginally so compared to actually winning warzones. CZ Massacre missions are almost always +++++ influence due to the credit rewards involved. Wartime is also one of the few times you can deliberately target missions against a lawful faction (via massacres), otherwise it's incredibly difficult to target negative effects against lawful factions.
It's doable now with the 24 hour timer (which, as I've already claimed, is already too long), but what you instead do is just ensure you have a stack of 20 completed massacre missions at the end of the war... once the war is over you just dump them out for major damage against your enemy's influence, and major positive effect for your supported faction. For a single player, it's a substantial effect, for a coordinated group, you might as well just kiss goodbye to that faction.
Again, banking missions is far more situational these days, but it still works, and would be made easier if you just extended hand-in times.
EDIT: As I also posited though... would be happy for longer hand-in times if a reward got pared back in that case to just credits.