lol, we specifically refer to the Domain as the Force in RR7. I don't think she's necessarily going to be merged, but since it's been over a hundred thousand years since the original firing of the Array, I have a feeling the large amounts of locally-stored information (stuff not in "the cloud" of the Domain or whatever) has slowly began to rebuild itself, and with intelligent instances of the Flood beginning to form, I don't doubt they were helping spur it along - especially since Gravemind was borderline Key Mind status when he took over High Charity. Considering how many times Cortana's tapped into Forerunner installations exchanging data (saving all the info from Alpha Halo's control room before eventually offloading it at Cairo Station, jumping from place to place in the Mantle's Approach, etc.) she's probably left a handful of "imprints" in the same way the Composer leaves behind data for "souls." Now that I'm thinking about it, I have a feeling whatever the Librarian did to the Chief involved some kind of latent connection to the reforming Domain, which would let him see these occasional flickers of Cortana, rather than him receiving patchy transmissions like the Cortana Moments in 3.
Being given "admin privileges" for the Domain could also explain why he wasn't killed by the Composer; depending on how heavily that cloud storage is used for the handling of all that soul data, Chief may have already been "logged in," and as a result didn't need to be forcibly added to the server to retrieve his information or whatever.
This is also why I partially think the new Promethean Soldier forms are so humanoid: they're either composed humans that have developed a physical form more familiar to work with, or the Soldiers have been made by the modicum of "imprinting" Chief has made in the Domain, meaning they're basically super-soldiers squared.