I can't answer all of your questions with certainty, however I can most of them...
2 No, it was on a Geschwader (Wing) level. That said, sometimes a Geschwaders Gruppen could be split up across different fronts, so some Gruppen may have carried them while others of the same Geschwader were wearing a different theater's colors. ex. in 1944 III/JG54 was serving RVG duty wearing the Geschwaders blue band while the rest of JG54 was in Russia wearing Eastern Front markings (yellow tail band, wing tips and patches under the engine).
3 Around mid-'43 until the end of the war.
4 Not a military tactician but I don't really know what advantage could have been had by knowing what units the aircraft came from, in combat at least... If they even knew what the bands signified in the first place
5 The bands were used on units employed in Reichsverteidigund duty. Once the markings came into use, aircraft not attached to RVG still carried their respective theater markings, whether they were in the Mediterranean (what was left of it) or the Eastern Front.
6 I can't answer that definitively however it seems a fairly trivial thing to protest when your country was falling apart around you, and tail bands had been in use for identification since the war's start. Many other identification systems had come into use soon before the RVG band system had come into use. The only time I've heard of of a wholesale protest of unit markings was when JG53 painted out the swastikas on the tailplanes of their 109's in response to being forced to paint a 'red band of shame' around their unit insignia by order of Hermann Goering. And that was during the BoB, much happier days than early '44.
7 If the Luftwaffe had amazing foresight...
The bands came into widespread use very early in 1944.
