1. EasyABC is a free, cross-platform suite built on the most universal ABC utilities (abc2midi etc.) However, you can use the online converter at mandolintab.net without installing any software at all. There's another online converter at Concertina.net, but it's buggy and waaay behind on updates; I don't recommend it.

2. EasyABC supports import of MIDI files (according to the Import dialog available from the File menu), but I've never used this feature. Generating good scores from MIDIs is so error-prone I find it easier to transcribe most music into ABC by ear. YMMV.

3. I don't know which .mus format you mean—I know Finale likes to use this extension, but so might other programs storing with an entirely different format. Anyway, importing from proprietary .mus formats is unlikely to be supported. But you can generally export scores using the MusicXML format, which might be reasonably compatible with your chosen ABC program. Sadly, MusicXML isn't well standardized, so not everything might be imported, or the import may just fail.

4. You can write the lyrics under the notes manually, using "w:" lines. (That's what I do.) There are a few special characters to learn to get proper alignment (specifically, asterisk, dash, underscore and tilde). This is actually simpler and more efficient to input and edit that the techniques required by other music programs I've seen. The ABC 2.0 standard docs describe how to add lyrics. There's also the "W:" directive for adding more verses after the score, unaligned. (I tend to use the less portable "%%blocktext" directive instead, for formatting reasons.)

— Artful Codger