CoH style mentoring would not work in EQ2. For one, they wouldn't have access to the zones. For another, it makes gaining access to the zones trivial if you allowed them automatic access. Lastly, part of the fun in playing is to get high enough level to see Rivervale or Runnyeye or Permefrost, etc, and it would decrease the fun if you could get there at level 10. Where's the challenge in that?
Um CoH sidekicks don't get access to zones that they couldn't normally get into either so it makes your entire objection moot. The real problem in SOE doing a CoH style sidekick system is that spells/abilities/ and items would need to be buffed in effect to an approbriate level of usefullness. That I'm afraid is the reason I see them for not doing it the right way, because it would be too much work for these guys to do. Look at PvP not being included...
Still they did do some of the work by reducing the effectiveness of items so I kinda have to wonder how much hard it would have been to go the rest of the way...