RTC News Feed Viewlets

It looks like you can prefix the URL with ${server-URL} to bypass the need to manually enter credentials.

https://localhost:9443/jazz/service/com.ibm.team.repository.common.internal.IFeedService?provider=query&user={$user}&query=_3rw4ENinEeCs86pFfCGK70

becomes

${server-URL}/service/com.ibm.team.repository.common.internal.IFeedService?provider=query&user={$user}&query=_3rw4ENinEeCs86pFfCGK70

Is there a way to prevent a user from having to manually put in their credentials in the News Feed viewlet on a dashboard similar to how the My Workitems Changes Feed uses the current user's credentials without them manually logging into the feed?

There is currently no support for having users authenticate using their own credentials, other than the internal mechanism you've found, which piggybacks on the already authenticated service request and only works for the current server you're on.

The following enhancement tracks the feature to allow users to authenticate with feeds:
https://jazz.net/jazz/resource/itemName/com.ibm.team.workitem.WorkItem/134487