I haven't tried a recent version of WinMX yet, but I know that eDonkey-style partial file-sharing isn't supported. Those percentages have to with with multi-source downloads eg. You're downloading 50% from User4, because he has the fastest & most reliable connection. The other users provide the other bits, but of course it doesn't matter from which user you download which part. You'll get the file eventually either way! FT-clients use a similar technique when dealing with multi-source downloads.
Somebody else could more accurately answer your second question, but I believe there are problems with doing so. Especially considering that each client utilises a different protocol/network, and the OpenNap servers don't all use identical indexing techniques. The forced update might actually provide the answer!
