Are you sure you did everything right before even starting working on your mod? I recommend checking out this tutorial made by Janne and see if you didn't forget anything.
The only thing about the rifle nade that needs fixing is the vet 2 range and (like all grenades) needs to not be able to be fired while suppressed by a machine gun.
Agreed, or nade range should be reduced heavily while under suppression.
When a P4 wipes a squad, it simply has to do with the soldiers being all clumped up together. If they would spread out the formations more this would be fixed out of cover.