@Setanta,
I agree about leaving as much of the Ss' work untouched as possible. In my writing classes, if I'm teaching about articles (a, an, the), subject-verb agreement or coherence in an essay, I would only correct errors related to what I was teaching at that time and let the others go. I tell my Ss that I am doing so, and that they shouldn't assume that everything I don't mark is correct. If many students have the same error, even if it's unrelated to the current lesson, I just take a few minutes aside and explain it as an 'extra'. They appreciate that approach.
Google can lead you to Harvard, NASA or the CIA. It can also link you to some loudmouthed know-it-all with a high school education. With experience, it's not so hard to discern the meat from the trash.
Google can be used as a corpus. As such, it's more reliable than Wiki, as a corpus isn't based on opinion, only frequency of usage. I googled 'more superior'. Try it. How many links do you get taking you to examples of it being used by language professionals or, for that matter, native speakers of English? If it were a widely-acceptable phrase, it should appear in journalism, marketing, political speeches, commentaries, peer-reviewed journals, etc. However, what do you get?
The main problem I had was in finding a source - any source - that specifically cited 'more superior' (out of the uncountable number of other possible double-comparative errors) in relation to a grammar lesson. I found two. Once is about 100 years old, and the other is
http://www.iolani.org/home.htm Given the time span, it seems pretty clear that 'more superior' has been considered an error by grammarians for longer than any of us has been alive.
No, the Iolani School isn't the OED. If pressed, I could look into how I might access the OED, but to be honest, I probably won't.