Can I use EXACTLY one copy of a card? For example, could I have one flooding or one light nymph or one standalone mark of earth in my deck? Or do I have to use EXACTLY two copies of everything I choose to put in my deck? (barring pillars)
Also, if marks count as pillars, does that mean you can use one copy of a standalone mark, or would you have to own two standalone marks to use them?
Yes, you can use only one copy of a card.
Marks count as pillars and are not limited to two or less copies.
As for things like stealing to get a third copy and mutaing to get a third abom, the primary theme is to stop direct generation (FFQ, Pharoah, Deja Vu, etc) and moving cards to the deck or hand (Reverse Time, Mindgate, Fractal, etc), rather than disallowing all cards that can provide a third copy.
Earthquake and Trident are not banned because EQ's effectiveness is considerably reduced with two copies, and Trident's is reduced because all PC is allowed.
