[Harv72b]

Before you do anything else, shell out the bucks for PokerTracker. It will do all these calculations for you, including adjusting your WR/hands per hour for multitabling. Well worth the cost and then some.

1) Multi-tabling generally decreases your overall volatility. If you are playing a good tight/aggressive game and not repeatedly making mistakes, you benefit from every hand you play. Therefore, the more hands you play, the more likely it is that you will achieve your "true" win rate; by multitabling, you increase the number of hands you play/hour, which decreases the luck factor as it pertains to you. Of course, this assumes that your own game does not drop off heavily when dividing your attention among several tables.
2) Accurate enough. But like I said, PT will do this for you.
3) It'll never happen. Seriously, the amount of hands needed to give you your "true" win rate/standard deviation is so huge that by the time you pile them all up, your gameplay will have improved and/or the conditions of your games changed enough that it will affect your figures. There's a math formula that you can use to figure approximate numbers once you've played a decent enough sample (tens of thousands of hands at least), but you'll never know your precise, "true" figures.
4) I honestly don't know. I don't pay much attention to standard deviation, which may or may not be a good practice. It would stand to reason that the lower your SD for a particular game, the lower your bankroll requirements would be.
5) Most people go with 300 big bets. I've never attempted to figure out "exactly" how many I need because honestly, I don't think it really matters. A winning player who is disciplined enough to step down/take a break if he hits a huge downswing should be okay with 300, but more never hurts.
6) Most likely, but I couldn't tell you what they are. You might try asking this question on the books/software forum.

In all seriousness, don't sweat the small stuff. You're playing with a healthy bankroll for your limit, and you're winning. It doesn't really matter if you can reduce your risk of ruin from 2% to 1.3% by adding 13 BBs to your bankroll. Instead, focus on improving your game (which will reduce your SD & RoR as well).