I think you can't modify the timers while mid-flight. What you can do in flight is:
- Select salvo or single release in the Blenheim
- Select individual salvo options (1,2,3,4, etc, bombs) and spacing (one bomb every X meters on the ground, obviously you need to set up the bombsight with the correct TAS and altitude for this to work) in the German bombers
- Select which bombs to drop in the Stuka (centerline rack, wing ranks or all of them)
plus arm them in all of the above cases.
If i'm not mistaken, the delay detonation fuse is handled under the load-out screens just like the ammo belting for the guns.
Also, the Stuka has a default 15 second delay to allow you to clear the blast. Not a big deal against buildings and static targets but something to keep in mind when attacking moving targets like ships or vehicle convoys.
For vehicles aim slightly ahead of them along their travel path. Ships move slower so you probably don't need to lead them as much, just aim for the bow and by the time they detonate they will probably be under the center of the ship.