1: Is it possible to make a MIRV that discharges more than one class of projectile at a time? I want to make a sort of heavy MIRV that spits out a bunch of decoys in addition to the real payload.
Technically it's in the code for MIRVs to do this already. 
Sadly I don't believe the secondary projectile can do damage, but if you look at the stock Hurricane MIRV projectile set you'll find both the projectile ("type_1_mirv"), the warhead ("mirv_warhead"), and something called "type_1_mirv_empty", which is by default commented out of the MIRV first projectile code:
"behaviorSpec":{"behavior":"MIRV",
...
"projectileSpec":"mirv_warhead",
#"emptySpec":"type_1_mirv_empty",
...}
If you uncomment that for your game, it causes the missile to spawn an additional "inert" projectile (the empty projectile file
is a missile but has the tag "NO_AI") upon splitting, which displays as if the initial propulsion stage of the missile carries on for a bit after it has ejected all the warheads. I don't believe it does any damage, but there probably isn't anything stopping you putting a non-inert projectile in the "emptySpec" for your missile.
Man, this actually sounds like a cool idea. I might test this myself.
