1) Can we add song lyrics from external sources and will they stay there whenever I play a song (BTW, where does it get stored e.g. my own machine?)?
You certainly can. You can open up the tag editor and paste copied lyrics to the lyrics tab. Or you can set up web links to search for lyrics manually. You can choose to embed the lyrics within the file or they can be stored as a text file for each track.
2) Can we suggest additional services? For instance, I want to add song lyrics from the Marillion.com web site as it is the official source of lyrics for their songs. Link: www.marillion.com/lyric.htm
You might be able to create a web link for it. Go to Preferences > Internet > custom web links. Look at how the default web links are constructed and adapt as necessary. For example, to create a web link for lyrics on Genius.com, I use
https://www.genius.com/search?q=$Replace($First(<Artist>),&,and)+$Split(<Title>,"(",1)
You may have to play around to get the syntax correct.