QIf I have 2 copies of this follower on my field, can I activate both their abilities in the same turn?

AYes, you can.

QWhat does "Add 1 to a Stack on your field" mean?

AIt means you put a Stack counter on an amulet with Stack on your field.

QIf I activate this follower's ability once during my turn, can I activate it once more during my opponent's turn?

AYes, you can.

QIf I play this follower's activated ability, then banish 10 spells in my cemetery to play Dimension Shift for 7 play points, does it instead cost 3 to play?

AYes, it does.

QIf I play this follower's activated ability, then play a spell for 0 play points with Merlin's On Evolve ability, does the next spell I play cost 4 less?

ANo. The effect of this follower's activated ability applies to the spell you play with Merlin's On Evolve ability.