Not every word JB rights is a conspiracy. Sometimes, he's just telling a story.
They were the least trusted. There is a difference. An incredibly deceitful person might follow your orders to a tee but you know that eventually when they get the chance they will betray you. Another reason to wonder if she had a Coin is that Jim said that at her age (around 100) having a baby would have required "help".. Those with Coin's don't appear to age.
I'm not even going there on the reproductive life cycle of wizards.
Nicodemus told Harry that he respected Margaret. If she had held one of the coins why didn't Nic say, "I respected your mother and worked with her from time to time, or "I respected your mother, we had mutual acquaintances."?