The proxy page included with the Silverlight API is a streamlined/enhanced version of the one included with the JavaScript API. Basically generating the request and processing the response have been streamlined for the Web APIs. Plus the proxy included with the Silverlight API supports secured services using tokens (long and short term) and http/windows authentication. The JavaScript API proxy only supports long term tokens.
1) Yes. As far as redirecting requests is concerned, the proxies are basically the same.
2) The Silverlight API proxy sets the content type of the request (if a POST) to "application/x-www-form-urlencoded" because its only been designed to support the use of a proxy (e.g. ProxyUrl) within the Web APIs. Since POSTed requests from the Web API to the proxy always contain text, this content type will work. However if you wanted to use the proxy to upload a file, you'd probably want to go with multipart/form-data. Of course you can try to set it on the fly like in the JavaScript API. Keep in mind you can modify the proxy at will; it only provides a workable starting point to support the use of a proxy in the Web APIs.
Ideally at some point in the future the proxies will be synchronized.