One thing I noticed is that your size 6 submissiles have a good range of 72m km, but you are not using that range with the separation range. As you currently have it the bus will need to get into very close range before it releases it's warheads. If on the approach the bus is hit and destroyed then you loose all of the warheads as well. At the tech level you are working at I would recommend setting the release range to about 50m km. This will leave plenty of fuel for catching up to a fast moving target while leaving you plenty of margin for error.
I realize you have 10 points of armor on the main missile, that is probably not going to be enough to keep your missile alive. It will only take a three hits from anti-missiles of comparable tech to statistically get a kill. Here is a sample size 1 anti-missile.
Missile Size: 1 MSP (0.05 HS) Warhead: 8 Armour: 0 Manoeuvre Rating: 94
Speed: 299000 km/s Engine Endurance: 1 minutes Range: 17.0m km
Cost Per Missile: 7.43
Chance to Hit: 1k km/s 28106% 3k km/s 9306% 5k km/s 5621.2% 10k km/s 2810.6%
Materials Required: 2x Tritanium 5.43x Gallicite Fuel x50
Development Cost for Project: 743RP
Please note that this size 1 missile is guaranteed to hit any of the missiles you have. Against the heavy ASM it has a 32% kill chance due to the massive armor. Against the Mirv missile it is a 44% chance. With the range of this amm it will have lots of firing chances against any incoming missile attack (approx. 10-12 depending on geometry) The missile ecm will help a lot against a lower tech enemy, but against one of similar tech it won't do much. The difference in ecm vs eccm levels is applied as a percentage penalty to the range of the missile fire control shooting at the missile. Let's say that the defender was cheap and only put on compact eccm 5 for his point defense. This would result in a 50% range reduction that the point defense missile fire control could actually fire on the incoming missiles. A .5hs fire control has a range of just over 22m km against size 6 missiles, against these monsters it is over 200m km. Net result is against the size 6 missile they would start firing at 11m km. Against these monsters it would be at 17m km, the maximum range of the amm.
In general the ecm/eccm involved with missiles rarely makes much difference once the two sides know what is going on. This is because it is to easy to refit a ship with a little larger missile fire control to get around an enemy having better ecm than you can counter. If your missile gets into energy range the ecm on board may end up making a big difference as there the difference is applied as a penalty to the base to hit chance. So even couple of points difference can end up resulting in a major difference to the hit rates. Unfortunately most races use a combination of amm and point defense beam weapons. Your huge missiles are just to easy to destroy at long range with amm.
For the mirv missile I would recommend ditching all the armor and ecm to add a larger engine, more fuel and 1 extra warhead. This will give you significantly more range and complicate matters for your enemy. If you know what the enemies missile range is for amm I would recommend setting the release range just inside of that. This will make them waste a salvo or two on the busses that will dispear as soon as the warheads are released.
Brian