1.
Ok but it's a software, not a library, right? And I haven't looked at their source code yet.
I think this software is modular so you can take just its libraries, and if you want to look to its source code, you can go to
https://modplug.svn.sourceforge.net/svnroot/modplug/2.
I'm not an expert but I heard that a good instrument bank was at least 10 MB. Others are either bad quality or not complete enough. Well, that's what I remember, it may be wrong
Roland GS soundfont (about 3 MB in size) is actually same as GM.dls in Microsoft Windows so you can hear it by yourself how good it sounds. Or if you may you can try Synthfont's soundfont (about 1 MB in size) by downloading it at
http://www.synthfont.com/SynthFontSetup.exe and play it or inspect the program folder by yourself.
3.
But even 1 MB is a lot, it would be embedded inside the sfml-audio library. And I don't even know if it can be embedded properly.
Okay, if 1 MB is a lot for you, so you don't need to embed it to SFML. You can distribute SFML as is, then just let the programmer load their own soundfont as you let them load their own true type font and render it with SFML. Because basically .mid for music format is same as .odt for document format.
4.
It would also cause problems such as "my music, played by SFML, is not the same as when I created it" -- because the sound bank was a different one.
It is same case as distributing game with TrueType Fonts. The result may different for every computer, (if the font itself wasn't distributed with the game) but the programmer have more freedom to choose they desired font to distribute within their games.
5.
Same problem, I have to find a library. But if I remember correctly, there was one (libmodplug?). Don't remember if it was good, maintained and had a compatible license though.
libmodplug is actually based on OpenMPT, so currently it is still maintained. It is very good library because libmodplug is used by many music player, for example Audacious to playback module music format.
According to its site, OpenMPT (since OpenMPT 1.17.02.53) is licensed under 3 clause BSD License that was compatible with SFML.