Firer Systems Down & SLAMs

In a pickup game at DundraCon this weekend, we had an unusual situation occur.
A SLAM fired at target vehicle. It took out the target & hit one of the
secondary infantry stands. When the chits were drawn for the infantry, a Firer
Systems Down was pulled. How do we work this? Since Firer Down invalidates the
shot, does a Firer Down on a secondary target invalidate the primary hit? Or
does it just invalidate that secondary & any following hits?

Also, I know this is just a game mechanic, but it seems strange that ECM & PDS
would prevent a GMS system from going down. Chits are only drawn if a GMS
missile gets past ECM & PDS. If they stop the missile, then they also
eliminate the chance of the firing system from going down. I guess this is the
same with all weapons, but for some reason,
I just noticed when trying to counter 5 GMS/Hx3 vehicles.

Thanks,