I don't know... I left Preston at the Museum, this go-round - went far enough to get the T-45, but never want back to Preston to finish the quest. Didn't bring him to Sanctuary, at all. Instead, I decided to live alone at RR, develop Abernathy farms, and connect Sanctuary, RR, and Abernathy via Provisioners. Now Abernathy is a nicely functioning settlement, Sanctuary is empty (edit: Codsworth is still there) but harvested for resources, and I live at RR with Dogmeat and am really loving life. No MM quests pestering me. Won't get to use bombardment, but never really used it that much on my last character anyway. Loving it so far, still haven't met or joined either BoS, Institute, or RR..... may get them all three going for the basic quests, and not pursue the MQ for a very long time...