The big deal here, whether by license or by spirit, is attribution.
It's not appropriate to copy and paste contributions by other users unless you properly attribute them to the person who originally posted them. Likewise, it's not appropriate to copy and post content from elsewhere (e.g., on the Internet, in a book/magazine, etc.) without properly attributing it to its original author/source.
So in the case of your edit, I think that was definitely the right thing to do, and I think it solves many of the problems with that answer. Please continue to improve answers like this one when you see them—we really do appreciate your efforts!
Personally, I would prefer to lead with the attribution (and I've edited the answer in question here to reflect that preference), but that's really of little consequence. The bigger point is that you must give credit where credit is due.
As far as the legality of quoting content that is licensed under another license in a post that is licensed under CC BY-SA, I can't speak authoritatively because I am not a lawyer. But I think this falls under the umbrella of "fair use"—we're reproducing a small section of the content for illustrative purposes, and providing a clear link back to the original source.
As well, I think you're probably right in noting that when something is quoted, it's an obvious indication that it's not the poster's original work, and thus (s)he does not have the right to release it under a different license. Quoted works retain their original license.
I haven't heard anyone complain yet, so I think that should be a pretty good indication that we're alright.