Adding a point in holy shock doesn't ruin anything, but as said, it is a wasted point.
Auras from items do never stack with auras from "hard" points invested.
There's no need to put points in that aura, since you will want to use conviction anyway.
Points in the resist lightning aura will however grant synergy bonus to auras from items, so you will want to max that one out.
Auras granted from runewords will never give a "greyed out" effect in the skill tree, those auras are just "active", but you'll never find them in the skill tree. (as in: you will never see the auras on a non-pally skill tab anyway, even though you can equip aura-granting items)