I think they apply differently when enemies have the same aura. However, Conviction is an exception, and why is of a complete mystery to me. Conviction doesn't affect its caster.
Yes, Mephisto's Conviction can be cancelled when a Paladin has a level Conviction > Mephisto's. The one from Mercenary and Infinity is too low to be boosted, and Mercenaries tend to die. Alot.