Unfortunately, the darkspawn spread corruption just by being somewhere, and Solas probably won't use them for the same reason Corypheus didn't: He wants the world to actually be habitable after he's done.
Maybe, but that assumes that
ELVEN MAGIC can't put things back in order when he's done with them. I've actually been thinking something similar to TC, really. I think the whole point of the Darkspawn/Blights is to break the Veil apart in the first place...considering that nothing can survive the Taint, ultimately, and the Veil gets weakened by death. Put enough Taint in a big enough area, and you could potentially start a full on fissure in the Veil that can be used to tear the whole thing down, perhaps.
Plus, bad things would probably happen if the taint got into the fade, and Solas wants it to become part of the material world again. Even demons avoid the Black City for this reason, if I recall correctly.
Well, to be fair, the Taint
came from the Fade. I was under the impression that demons and spirits "avoid" the Black City for the same reason Mages do...even though it's always visible, you can't actually get to it.