Crocus could have been lying about his time spent with Laboon. Granted, they did spend a significant amount of time with each other for Crocus to build the waterway and the island-boat in his stomach, but he could have gone to sail with Roger and then COME BACK.
Nico Olivia is a perfect candidate. She left the island when Robin was 2. That was 26 years ago. Since Roger was executed 4 years later, that gave her 4 years to sail with him. Plus, her husband could have been a member of the crew as well.
D. Ragon is definitely a crewmember. There's no real question about that.
Tom and Kureha are unlikely, but he did need a shipwright and a doctor. Maybe Roger devised a way (to protect his crew) that everyone would forget who was in his crew. That upon his death, they would be able to live normal lives without problems (Well, except for D. Ragon), the most likely idea would be staging their deaths. Perhaps D. Ragon refused to do something as base as falsely killing himself (ditto with Whitebeard) and kept his bounty, which would explain why he's "The most wanted man in the world".