1. If you want to, you can. But what counts as a person and therefore ATM can only be RM is stricter than most micros, and you can't send a file/similar RM if you have it RF elsewhere.
2. RM doesn't have to be exclusive to Alamy, but as above, any file you're submitting can't be on/have a similar anywhere else as RF.
3. I can't work out why my files sell - or don't sell! - other than it happened to be what someone needed at the time. It all seems very random from my port. Others might have different experience. All my files are only on Alamy, and all are RM. However, Alamy doesn't require RM exclusivity, you can have the same files elsewhere as RM.
4. Alamy is planning to offer RF-editorial

.
AFAIK, they haven't announced when this will be or any other details as of now. (If anyone knows otherwise, could they post a link, please?)