I have started attaching spies to my armies with important generals after one of my generals was assassinated.

Do spies reduce the chance of assassination when with a general in the field?

I am also curious if attaching priests or bishops to an army with a general has any advantage. Would a bishop attached to an army prevent heresy attacks on my general? Would having a priest attached give any piety characteristics to my general?

Are there any other types of agents useful to attach to general's armies?