Let me start by answering your questions in order:
1.) Exams have an expiration date set by Microsoft. These dates are normally set when updated or newer versions of technology makes the current exam objective obsolete. With that said, the certifications you mentioned will not expire (even after the required exams for those certifications retire). Once you achieve the mentioned certifications, they will not expire. The new generation certification (MCTS, MCITP. MCPD) have a retirement date. This date typically coincides with the termination of mainstream support for that technology.
2.) Yes. But be aware that some required exams are also electives. In other words, the 70-620 exam can be used to fulfill the client operating system portion of the MCSE (OR) count as an elective; but not both.
3.) No. However, it is highly encouraged that you stay current with newer technology.
4.) n/a
5.) Normally you will find discounts for exams in the back of Microsoft Press Training Kits.
6.) Proficiency in the tested material is the best material. However, Microsoft Press books along with practice test material produced by Measure UP, Self Test Software, and Transcender are great resources. Free whitepapers concerning tested material can be located on the Microsoft TechNet sites.
Michael D. Alligood,
MCITP, MCTS, MCSA, MCDST, MCP, A+, Network+
The I.T. Classroom Blog
Start. Research. Plan. Perform. End. Test. Evaluate. 
