So there's a backdoor to inject zimlets manually, but I don't encourage that unless it's the last resort. In your situation, you can go to
Zimbra offers Open Source email server software and shared calendar for Linux and the Mac to download the installer again, and run it. It will just uninstall/reinstall your current version, and most likely fix the url zimlet issue in the process.
In terms of prism performance, it is definitely not cheapest app to run. But I would say it's on the par with ZCS UI in firefox.
For the haning issue, there's a known problem in 1418 that can hang the UI if you have mountpoints for sharing. This will be corrected in the next release.