Here are some relevant threads:
Critical hits, math vs reality
Missile Vs Shield, math vs reality
Also an older thread:
Missile Vs Shield, Dom2 vs Dom3
Was the missile formula actually changed? If the parry value isn't doubled but just taken as-is, well, that'd explain many things.
I'll test Swarm vs Arch Devil once I get home. Does it matter what weapon I give him? I'll also try it with an Ice Devil with a burning pearl, to see if it's some bug with the attack bonus from Fire or something similar.