Yep, they can raise the timer the important thing is immediate confirmation like you said, real life will always come first.
Increasing the timer length acts as a disincentive to casually disconnect, but moving the confirmation allows those determined to disconnect to do so.
In an example of rage quitting (ie not stigbob's example) the quitting player probably would be psychologically better off if they don't have to sit & watch their ship get pounded to smithereens, they can just walk away & come back once they've calmed down a bit.
Should the player using the combat timer (for any reason) be forced to rejoin the mode they left? Increasing the timer also increases the likelihood of the player returning to a rebuy screen (where the ship is no longer in danger) but if they were not destroyed...