I'm sure Bishop or Aaron will be along shortly to give you some better answers, but, short, quick answers...
1) You sign up on
http://www.catalystdemos.com/ under "Enlist!" That should give you most of the basics.
2) Nothing. The Commando program was FanPro's demo team, and is now defunct. Now, you become a Catalyst Demo Team Agent, and an Agent is simply someone who's registered to run Shadowrun events (Demo's and Missions) and represent the Demo Team. Generally you do this by registering a Firebase (Usually a Gaming Store, or some other public place), and publically listing and running at least a certain amount of events per year (I'm not 100% sure what the minimum is to stay an active Agent). You then get credits for these events, that you can use to get swag of some sort. I only do Convention work, so I'm not 100% sure how the "regular" Agents and Firebases work exactly.
3) Yes, you need to be a registered Agent. But again, as with #2, I'm not certain of all the details.
4) I'm not entirely certain about the SRM2 Modules. I want to say "yes", although we're encouraging SRM03, since that the current, active campaign and most players will be looking for these, but I'm not 100% certain.
5) Re: Time. There's a Calender that you use. Each run takes up one "week" on the calender. You also use the calender to track "downtime" stuff like getting gear via Availability, training up skills and attributes, and paying lifestyle. So let's say you play SRM03-00, that gets marked as the First Week in January on your calender. Then, your character goes looking for some APDS ammo, which after calculations, will take you 2 weeks to get. You decide to wait to get these before you run again. So when you play SRM03-01 next, you would mark that on your calendar under Week 4.
6) Re: Multiple Characters. I'm not entirely sure I understand this question. Do you mean "Can a GM only use a module once, then he has to buy it again to use it again?" If so, then no. It would be silly to try and even enforce something like that, even if CGL and the Demo Team wanted to, which they don't. If you're talking simply about a player going through the adventure more than once with different characters, then also, no. Though playing modules more than once is a bit boring, IMO, and if the GM knows that one or more playes has been through it before, he may want to ask the player to step back from the decision making process so they don't spoil it for the other players.
I know myself and Caine_Hazen ran through 03-00 and 03-01 at Conventions last year with our characters. We joined a new campaign recently, and we're using these same characters, but the campaign started with 00 and 01. So while we played, we stepped away from the decision making process for the most part, since we were somewhat familiar with the events, and since our characters had already been paid for the missions and had gotten Karma and other rewards, we basically did the first two missions for free, just to let the group catch up. We're eager to get back to the game now though, becasue we're hitting 03-02, so we'll start getting money and Karma once again (The GM had to take some time off as his wife just had a baby).
Bull