1) If I buy my domain through WordPress, can I purchase it for more than one year - cheaper over the long run?
Yes you can purchase a domain mapping upgrade for more than on year.
2) If I buy my domain through WordPress, do I get a .com email?
No but you can set up email by using Google apps.
3) If I buy a .com domain address and after a year I decide not to renew it will my ...wordpress.com address still work?
All of the links to the domain which will have been indexed by search engines will be rboken. When clicked the visitors will experience a 404 (page not found). The content will still be in the .wordpress.com sub-domain blog however, the search engines will have to index it all again under the .wordpress.com URLs. Google clears their caches every 3 - 6 months so what you will create is a mess of broken links while the content is being re-indexed.
Also note that failing to renew a domain name means you no longer own that domain name. There is no guarantee you will ever get it back. Among other things and it can be auctioned of or purchased by a domain name squatter who will want you to pay big bucks if you want to get it back.
4) Will my ...wordpress.com address still work even if I register a .com address?
Anyone who clicks the .wordpress.com blog address will arrive on the domain if domain mapping is in place. if not then there is no transfer to the domain URL.
5) If I register a .com address with another company, is there any cost for adding it to my wordpress.com site?
If you register a domain name elsewhere you still pay wordpress.com for domain mapping of an existing domain to your content in your .wordpress.com sub-domain.