Well reasoned argument, but you leave out one important possibility. I don't think that an alien civilization will colonize Earth because Earth is some special, valuable, interesting place. They will colonize Earth because they will colonize all of their light cone. Let me try to explain how and why would they do that.
The why question: The relevant buzzwords here are computronium and negentropy. Basically, for any sufficiently advanced civilization, there is only one valuable resource: computation. (If this sounds implausible, just try to imagine how much of our existence will be spent in virtual spaces in 50 years from now, and then extrapolate another 100 million years from there. Upload is another relevant buzzword.) So on a conceptual level, and strictly from an outside view, you can imagine an advanced civilization as a process to build the largest possible computer. And that requires a huge amount of space.
The how question: Neumann probes are already a well-known possible method for totally terraforming a region of space, but I think much more effective methods are possible, too. We don't know any laws of physics that would be violated by an expansion speed converging to the speed of light. I imagine this as a turbo-charged version of the terraforming method described in "His Master's Voice", with teleportation instead of the gentle catalysis.
In fact, I believe in a 0-1 law for the expansion speed of civilizations. I argue that there is only a very short timeframe in the life of a civilization when their sphere of influence is already expanding, but not yet expanding with the speed of light. Note that this idea is an (admittedly somewhat contrived) solution to the Fermi Paradox: If they are before this short phase transition, they can’t be observed with current human technology. After the phase transition they can’t be observed at all.