This be tackled systematically, by group of some kind- Child, or even simply alphabetical groups?
Joe mentioned the work by Alan and Malcolm on the Oxford Book of English Folk Songs and a start could be made here- it might be a good test because mostly only one version is given, and there is much to be added (see recent additions to thread on "As Sylvie Was Walking," 17057: a href="/thread.cfm?threadid=17057"> Sylvie . Songs of the same cluster could be grouped and referenced. Also verses of "Sylvie" cross over to "Green Grow...", etc.
Another group is the thread with histories, started by George Seto and Katlaughing, also, I think, mentioned above.

I am trying to recall examples where a song or critical information has been taken from a website and not credited. I can't so they must be few. A clickie or an acknowledgement of the website, usually both, appear in the Mudcat postings I can remember. For the thread on "Any Old Iron," I didn't mention a site with a garbled version that has Derby Kell changed to Darby Gel but no one would seriously quote that site.

I agree that there is little point in re-doing a good site such as Contemplator. On the other hand, some primary sources of material such as the Bodleian collection will have to be used (and many of the broadsides are hard to read and should be reproduced in fair copy).

Material post-1920 should be left for the time being unless directly related to an older version. This might save a lot of argument about copyright, etc. There is more than enough older material to chew over.
Finally, I agree with McGrath that there should not be two separate threads. There will be questions and speculations as the thread develops; unnecessary material can be edited out of the "definitive" version.