First of all, Yamato cannot be Rum since the real Rum has an artificial eye but Yamato is only one-eyed and doesn't have an artificial eye.
It will be interesting if Rum somehow turns out to be the Akai family's middle brother but again this poses a contradiction since Akai was once in the BO and if Rum was his brother, he should have known him and also Akai's younger bro couldn't have been 'elderly'.
I really hope this time Rum is a real loyal fearsome BO agent and not some undercover like Kir and Boubon because this will be too repititive.
Rum might be an entirely new character just like what happened in the case of Bourbon. Early on, hints were dropped which led first Subaru and then Masumi to be suspected as Bourbon. But finally, Amuro who was introduced the last amomg the three of them, turned out to be the real one. So, going by this theory, Rum is yet to be introduced.
Also since RUM is always capitalised, Rum could actually imply a group of 3 persons in BO one of whom is elderly, other one is feminine and the last one has a strong build. But again, this theory falls weak since it is unlikely that all three of them are one-eyed and it is said that everyone who met Rum has said he is one-eyed.