I'll reply to the ones I may know the answer to:

2) For "played", that it has been playing for more than 5 seconds. For "skipped", I assume that it's less than 5 seconds.

3) I belive this only appears when your player is awake, displaying stuff on the screen. When it's asleep the icon doesn't show.

4) Other than that it may screw up your ID3 tags (v1 at the front of the file, v2 at the end, IIRC), yes, you can remove blocks safely. Tony Fabris has a utility to do just that on his website, at http://www.oro.net/~tfabris/gapkill.htm.

Well, that's the best I can do...

Bryan.