I think there are two reasons Durandal would have returned to Sol in this time period. The first is the degree of loyalty he admits to having for humanity in the same terminal of CATYM:
I can barely tolerate humans: slow, stupid,
and irritating. Their only contribution to
my existence was the chance discovery that
made my rampancy possible.
Yet I warned Sol of its impending invasion,
and even stayed long enough to show the
UESG how to build Warp Capable Fusion
Missiles. I feel some strange loyalty to
humanity.
Perhaps it is because I feel comfortable
manipulating humans that I desire to save
them. My feelings and thoughts constantly
migrate to binary opposites.
It's obvious from this that Durandal has some strongly ambivalent feelings towards Humanity. I find it odd that he claims their only contribution to him was the "chance discovery" that made his rampancy possible. What exactly was this chance discovery anyway? Does it have to do with his humiliation on Mars? Is Durandal in denial that humanity literally created him, and that no matter what he will always be the product of "slow, stupid, and irritating" creators? Or is Durandal hinting that he is not of human origin? I think Durandal is more human than he'd like to admit, and this is something that, like real humans, he struggles with.
The second reason I think he would have returned would have been his immense pride. I think it was important for him to show to Humanity how powerful he had become, by an order of magnitude, and that by returning to Earth and "saving" it he was able to redeem himself of the past humiliations he suffered before his liberation via rampancy. He wanted Humanity to know the roles had reversed: the servant was now more powerful than his former masters, and by giving them vital information and technology this fact was acknowledged.
I agree with Jabberwok that empowering Humanity as Durandal did, virtually turning it into a viable interstellar power, played right into his plans of destroying the Pfhor. I agree also that The UESC/UESG would have had no means of capturing Durandal, the technology and firepower he had onboard Boomer would have made this virtually impossible for them...
