Unguided missiles: Non-homing. Unguided missiles require the target to be illuminated by an active scanner. If contact is lost or the controlling firecon is destroyed an unguided missile will self destruct.
(self)Guided missiles: Homing. If the primary target is lost a self-guided missile will switch to on-board guidance and continue to the point the target was last seen. These will retarget on anything that enters on-board sensor range.
A) Firing a guided missile at a waypoint and then deleting the waypoint will cause a guided missile to home in on anything between it and the waypoint that enters sensor range. Please note that missile sensor ranges are very short, so this requires very careful aiming, luck, and an unusually large on-missiles sensor. This is not a recommended tactic.
B) It makes them homing. See above.
C) The drone engine option has been replaced in 6.x by custom engine design. You can recreate one by designing a large missile engine with a low power modifier.
D) If 2 multistage missile are launched in the same salvo then all the 12 payload missiles should also be in the same salvo.
E) If you achieve this by cycling between multiple missile launchers with 15s+ reload times then yes, it can be (slightly) more effective at penetrating AMM defenses. If you have 4 or more 5-10s reload launchers then no, restricting yourself to only two launchers firing will not help unless you are only trying make the defenders run out of AMMs. Either tactic will be much less effective against beam point defenses.
2-A) The simplest and most reliable method. This should be longer ranged than a similarly sized thermal sensor. While EM sensors have even longer ranges at detecting enemy active scanners, the AI tends to leave their active scanners off until they know you are within sensor range.
2-B,C) This will only work if the enemy fleet is not moving.
2-D) Effective. More effective than A if you detect the enemy on passive sensors before they detect you, less effective than A if they find you first. The fighter or small ship must be able to light up the enemy while staying outside of enemy antimissile range or Bad Things will happen.
51m km sounds closer to medium than mid-long, but should work fine for a fleet of missile boats. I'm assuming a missile boat is <3,500 tons. If your missile boats are larger you may want a longer range.
Edited to clarify previous terms 'guided' and 'unguided' as 'homing' and 'non-homing'