Hi Latha -
This is a pretty big question with not too many specifics from you to help craft a focused response. Any answer one could give to questions 1, 3, 4 would be therefore be so general they'd be useless to you.
I'd recommend you take this conversation up with a Sales Account Manager. He or she can address question #2 (FYI, we don't do concurrent licensing) and speak to the rest of your questions, too.
I know this doesn't help a whole lot, but you really need to talk to someone versus attempting to do this via forums.
100% agree with Russell. Talk to a sales rep.
For me the install was easy. What takes me less than 2 hours to upgrade (including VPN in and making a backup) our Cognos admin takes a weekend and then some...