A few things if I may. I'm no one's idea of an expert, I'm not a copyright admin, etc., but I have learned a few things and at least guessed at a few others that maybe some others can confirm.
Just because a scan turns up in a library does not mean the work is out of copyright even in the country in which that library is rooted. Mr. Farrington @ Sibley has informed me, for instance, that Sibley Library is allowed , by various library-related exceptions in US law, to host scans of works that are in copyright in the USA. These exceptions apply to institutional libraries as recognized by law, by the way, not to IMSLP - having the word "Library" in the name is not alas sufficient!
The mixed guesswork and obvious fact etc. on my part: Just because a work has been scanned in by a European library, or is in your collection as a purchased score, or appears in WIMA as a scan, does not mean it can be imported here with no further questions asked. A number of manuscripts scanned by European libraries have been published in editions or in facsimile fairly recently (say, between 1960 and 1999...), I gather, some perhaps un-recently enough to qualify for the 25-year gap rule in editio princeps... don't ask... but that might make these manuscript scans public domain in some areas and not others. Some domains have very complicated legal rules with regards manuscripts when this happens, depending on the composer, when they died, when that publication was, ...
It might be a -really- good idea ... maybe this should be said more strongly... before uploading to IMSLP... to check, say, worldcat.org , to see if the manuscript has or hasn't been published in the last 50 or 100 years, before uploading a whole batch of manuscript scans or of typesets under the assumption that the music is probably OK. Hurry is not one's friend. In my humble opinion.
I know. I'm repeating myself, and some of the qualifications to what I just said have been made by Carolus and others who do know better than I, in previous related threads. Apologies. (I think I can mostly here stand by the last and first parts: I'm no one's idea of an expert, but hurry -is not- one's friend. Hrm. Nor mine, apparently.)
-Eric, will probably have to edit this but sending it off into the world anyway *patpat*