Why not combine both options? Break off contact with A and tell Z everything, including the fact that you think you're poly but love her too much to force her into something that would make her unhappy, especially during this sensitive time, and so you cut A loose rather then get into a sticky situation again. Let her react to that and give her input. Maybe she'll be grateful, maybe she'll be mad that you brought it up at all, maybe she'll reconsider poly and do some reading, who knows. But you'll have been honest.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.