If you think about it, we only have the word of the Kelvans the Enterprise met that the Andremeda Galaxy was becoming uninhabitable.
Yeah, they are refugees with ambitions of conquest. Hmmm.
IIRC -- though this may have been 80s fannish speculation, come to think of it -- the Galaxy project from McIntyre's novelizations (Flynn's Magellan(ic Clouds), the Andromeda, and the M-31) was intended to make contact with the Kelvans on their own ground. A propulsion test project -- doesn't McIntyre actually refer to it as "transwarp" in her books -- perhaps with Kelvan assistance isn't impossible. Given the scale of distances involved and the whole test project nature of the endeavor, anyone signing up for the mission knows they stand a good chance of not returning to Earth alive. Of the three Galaxy ships built, I wouldn't be surprised if all three were total losses. The only thing we know, since McIntyre never returned to this, is that Flynn's ship surveyed a supernova in the Andromeda Galaxy.
At the very least, in my mind (just for fun) the Kelvans are an aspect of the expeditions to Andromeda. I don't recall if McIntyre says the Galaxy ships are transwarp ships, but maybe they are implied to have drive systems of similar lineage to what is in the Excelsior. I also came away from the novelization that Excelsior is slightly more advanced than the Galaxy ships. So the Galaxy ships are maybe no-frills scout ships, while Excelsior would do long term patrols in Andromeda the way Enterprise patrolled the frontier during the original series.
Wasn't Vance Madison the one that Carol Marcus had a relationship with, and she tried to find his family on Earth after he died, but he apparently somehow managed to live completely off-the-grid and didn't appear to have any sort of existence before the Genesis Project? I was always fascinated by that -- a 1980s Star Trek novel already exploring the idea of people who didn't "exist" in a surveillance society.
From memory, without checking, Vance Madison was raised in a fairly normal, loving family, and he had a gentle personality, and he was the one in a relationship with Carol. Del March met Vance when they were young boys in a neighborhood park where Vance lived. They became fast friends and grew up together, and Del "adopted" himself into the Madison family. Del is said to be a little wild and occasionally indulges in moments of dark anger, which combined with the hints that he was born within a bad element of society, I was left with the impression that he suffered from early trauma that his upbringing with the Madison family mitigated against. My recollection is that it is all but confirmed that Del ran away from or was on the run from wherever he originated from, and he chose a totally new name for himself (maybe to disguise his identity, or maybe because he was never given a name, or both).
And yeah, that's the really fascinating part, the mystery of a boy who ran from a seedy undercurrent of the society of the 23rd Century and became a respected scientist alongside his lifelong best friend and adopted brother. What aspect of society got missed in the 23rd Century, stayed in a dark corner?
I would have loved to have had a book that was about Del and Vance, with hints of several possible places of origin for Del (left for the reader to decide which they prefer or reject). With details about the development of their other crowning achievement, the computer game Boojum Hunt.
It's such good background information, and that's what makes me say that I have mixed feelings about it, that it feels a little morbid.
I have always wondered where Diane Carey's Piper novels are supposed to go. The continuity references in them are a little wonky. I know they're supposed to be FYM stories (which would have been explicit had her Lost Years lead-in, "The Federation Mutinies") been published, but other things in Dreadnought! make me wonder if it was meant to be closer to Star Trek II.
With both The Entropy Effect and Dreadnaught! as late 5YM novels, the turn around time for Mandela Flynn going from being the Enterprise security chief to captain of the Magellanic Clouds would be quite compressed. It's not impossible that Mandela Flynn was involved in a career-defining mission that got her a field promotion, where she was able to stay afterward. And then she remains as captain of the Magellan for something like 10 years or so, when she is first mentioned/introduced in that position in the novelization of TWoK. There are some explanations that can make that work, though.
It's too bad that Diane Carey wasn't able to do By Logic Alone and The Federation Mutinies. Although the combination of Dreadnaught and Battlestations are heavy with conspiracy, adding an epic book to that sequence would have made the Federation of the 80's novels appallingly untrustworthy. The Rittenhouse scandal, and it's aftermath as described in Battlestations felt like it cast a formidably long shadow just with those two books alone.