Let's see if this is the answer to your question.
No, you can not override the request parameters values neither reuse these parameters as variables. When you use the c:set tag with the attribute scope="request" you are setting request attributes and not parameters. As a proof, we know that a JSP "becomes" a
Servlet, so check the ServletRequest API in this link
http://java.sun.com/j2ee/1.4/docs/api/javax/servlet/ServletRequest.html
There, you will see the public methods getParameter() but no setParameter(), instead, there exist setAtribute() and getAtribute, that is what you are really doing in the code you posted inside the first message.