MuseScore runs natively in Linux; no problem with MIDI output that I know of. Last time I installed it, it was able to import ABC only via a web service (!) which ran abc2xml on its server - not what I would use. Rather, I run abc2xml.py on my local PC and load its output (MusicXML) into MuseScore, Sibelius, or Finale etc.

Either way, abc2xml is extremely useful and thus likely to be the main authority on ABC syntax. It claims to agree with abcm2ps and the original language definition in most aspects, but does not mention Five Line Skink etc.

Note that there is also a converse software xml2abc.py by the same author. Thus we can use MuseScore to edit music, export it to MusicXML, then convert the result to ABC to post it to Mudcat etc. Still, a slim editor like ABCExplorer has its merits, for those quick hacks that ABC is typically used for.