<I've snipped out a bunch of stuff. I will be addressing a few of your points, but I will also be attempting in this post to disambiguate our disagreement. It feels like we may be arguing past one another on a few particulars and I'd like to verify if this is so.>
Addressing points:
- can manage to create robust and self-sustaining space colonies to begin with (imagine, for example, how incredibly fragile and dependent a pioneer colony on Mars would be);
- doesn't destroy itself in the process of reaching a space-faring level of technology (we humans seem to be well on our way to meeting this fate);
- doesn't destroy itself after having reached a space-faring level of technology (e.g. inadvertently triggering a supernova after an attempt to directly harness the power of a star goes awry);
- doesn't succumb to some kind of interstellar super-plague as a result of its colonization efforts;
- manages to evade detection by advanced genocidal ETs who might perceive other space-faring lifeforms as an intolerable existential threat; and
- doesn't evolve into other species as a result of space colonization (in which case we have to ask whether that species has really given itself a hedge against extinction).
These are all pretty much selection events.
The supernova one is just silly. If a species had gotten to the point where they had the resources and expertise necessary to mine a star, there's no reason to expect them to biff it that badly. Of course, I'm a little confused as to why one would mine a star for energy when they radiate so much for free, but, yeah, really, this is silly enough that it's not worth poking holes in.
Speciation is pretty much guaranteed once colonization beyond solar systems happens. It's just not feasible to do any substantial mixing of gene pools over those distances. But I don't see that as an objection to interstellar civilizations. There is no reason a civilization cannot be composed of multiple related- or unrelated- species. It would probably work better if they were related.
However, in the nearer term there are formidable nearer-term selective pressures against space colonization (e.g. the total lack of a breathable atmosphere anywhere else in the known universe;
Er.... what? How can you justify this claim when we haven't gotten around to checking most of the known universe for breathable atmospheres?
the inherent cultural and political instability of human societies).
Political and cultural instability is why I favor one-shot colonization over long-term SETI-style missions. It's a lot easier to keep a group of us on track long enough to launch something big and exciting than it is to keep us on track long enough to be heard or to hear extraterrestrials with our modern equivalent of strings and tin cans. Also, there are non-ET-specific reasons to want to colonize other worlds, but that isn't the case for SETI-style stuff. If the colony works (and there's no guarantee that it will) you've established a new culture based upon space colonization. But, yeah, it's all just a shot in the dark. I'm only ever talking probabilities. It's all we have pending more real information.
All previous human experience of colonization has involved the colonization of places that already had bountiful complex ecosystems and - in many cases - helpful indigenous people. Without the first factor, human colonization of new lands would have been a non-starter right from the get-go; without the second factor, human colonization in more recent times would have been much more difficult and probably far less successful. Absent these two factors, space colonization is so different - both in terms of quality and degree of difficulty - from colonization as we know it, that it hardly makes sense to even call it "colonization".
Nope. "Colonization" is exactly what it is, in the purest sense of the term. And, in principle, it's pretty much the same. All that changes is the magnitude of the endeavor.
__Now for some disambiguation__
The basic issues:
1) Are there other life-bearing worlds?
2) What is the probability of intelligence evolving on those worlds?
3) Where are all the ETs?
Given issue (1) I believe we may be in disagreement. I am comfortable with the idea that there may be other life-bearing worlds. We've observed enough precursor molecules in space that it's even reasonable to assume that extraterrestrial life could use many of the same amino acids with the same chirality as ours. Whether that is actually true remains to be seen. All it would take is a planet of sufficient composition at a sufficient orbital distance from a star for life to happen. How do I know? Because it happened here.
Regarding issue (2) we may be closer to agreement. I believe we are both dissatisfied with the relatively optimistic assessment of how likely intelligence is given an ecosystem. I also suspect that our dissatisfaction may be divergent when it comes to specifics. My own reasoning hinges on my pet hypothesis that human intelligence is largely the result of runaway sexual selection, which would make intelligence like ours quite a bit less likely than if it were a result of less specific selection pressures. That, and issue (3).
Regarding issue (3) we may be really quite close to agreement indeed, but still differ on the specifics. I feel pretty comfortable stating that the fact that we haven't been contacted by ETs is indicative that there aren't any around. Also, with my particular sort of reasoning, the vastness of space isn't as big a factor in terms of why there aren't any ETs around. If conditions in the universe select for colonizers, then, given enough colonizers, we should expect to wind up with at least one really successful group of colonizing species which will eventually get around to colonizing in our neighborhood regardless of where in the galaxy they started out. And there's no reason not to expect that a really, really successful group of colonizing species won't just spread back and forth through the galaxy, evolving and speciating for as long as there are habitable worlds, and that this sort of thing can go on without any sort of central planning or coordination. I'll not go so far with intergalactic distances. That's vastness that boggles my mind sufficiently that I'm inclined to leave it alone.
As far as SETI-style contact, I'm really dissatisfied with it for a number of reasons, but I can collapse those reasons down to a nice, simple umbrella objection: It presupposes aliens which are too much like us, or perhaps like better versions of us. I prefer to leave a lot of room for the possibility that aliens are more alien than that.
by 
I'm all for making our own alien species.
