1) Your very first question all those years ago is one I am still wrestling with. I like the idea of a spellcaster casting spells on a possessed body and then jumping back into his gem and possibly another body. The potential for abuse that makes me uncomfortable is a caster being able to put a lot of personal buffs on a minion, and then sending that minion off to fight. A wizard could magic jar putting Shield, Expeditious Retreat, and Mirror Image on his minions, whether they were living mercenaries or intelligent undead. Putting personal buffs on minions is a pretty big restriction to get around, but it is a 5th level spell with some significant drawbacks so I tend to waffle on this point.
2) My feeling is no. In the receptacle the only actions available to you is to sense the life forces around and attempt to take them over.
3) My take is they would be disoriented and confused, but aware that their soul had been imprisoned or in some way removed from their body. Spellcraft checks to learn more.
4) I would probably rule that the caster could pick from the most powerful auras (randomly determined from all the 7-10HD creatures in your example), the least powerful (a randomly determined 1-4HD), or the middle (randomly determined 4-7HD). Partly that's to keep it simple, partly because it seems to me that since the auras have no identifying features other than relative power and you seem to have you no spatial awareness, you wouldn't really be able to keep track of specific auras.
5) My take is no. If the magic jar ends and you ain't go no body to go to, you would be slain.
6) The easiest answer would be no, that you can't have two of the same spells with the same effect active at the same time. A more interesting answer to me would be to allow the caster to keep casting magic jars to jump from gem to gem to extend the duration, but that's a fairly wooly ruling. I wouldn't blame a GM for wanting to keep things simple.
7) I'd say no. Referring to the rules on extradimensional spaces, it seems to me that while a space that does not exist in any dimension might not be an "other plane" neither it is your plane. You could store your body and the gem in a portable hole or bag of holding, but to have access to it, you'd need to take the body or the gem out.
I'd say the best get-around to the range restricition of Magic Jar (at higher levels anyway) would be the Clone spell. Once you've prepared your Clone, you put it in the safest place imaginable. Then you go off, Magic Jar into a host and do what you do best. If the unthinkable happens and you are unable to get back to your body (or it is discovered in it's vulnerable state and hacked to bits by adventurers with no sense of humor), your Clone comes online. Not without cost, but better than being dead. It certainly ups your unkillability quotient.