Media Center seemed a bit slow especially resizing windows and dragging and dropping songs. I decided to turn off the Pi and have a look at it again when we got back.
To some extent, this part is expected behavior. The Pi is not particularly computationally robust, so any GUI intensive activity will be a bit slow, especially over VNC (if you're using a virtual interface still). The best way to interact with a Pi instance of MC (once setup) is through an android or ios remote (like Gizmo or JRemote) which offloads some of the burden to the other device, and the next best is to use the Pi as a library client of an MC server on a more robust machine, and third best is to use the built in views to navigate rather than dragging and dropping.
Window resizing in particular has been a bugaboo on JRiver for Linux in general (not limited to the pi) and can be a bit flaky. One tip is to hold down the alt key while doing the resizing. To be clear, total freezing is not generally expected behavior, but slowness in the GUI is.
This is where the problem started. When I turned the Pi back on to check out my handiwork, it was not able to play any files from the library.
Can you post the error it throws? Did you take steps to mount the NAS in a predictable location as suggested above? My hypothesis is that the Music folder is being automounted by the OS in a way that doesn't produce a consistent mount point (analogous to the way windows handles removable media), so the database that MC built up now points to an old mountpoint that is no longer valid. The easy fix there is to mount your shares at a predictable location on boot so that the MC database will always point to the same place. That would also rule out some other potential issues.
I've tried to delete and reinstall the library, but Media Center keeps freezing.
I'm not sure what exactly you mean by "delete and reinstall the library"; do you mean you're making a library backup and reimporting the library as a whole? Or do you mean you're clearing the library and running an import to bring in the files? Are you trying to delete the only library on the machine? If so that might not work. A better approach is to "clear" the library, or make a second one and then delete the original.
I'm also not sure exactly what you mean by "freezing"? Does the whole UI lock up? Does a blocking dialog box spawn and never go away? Does MC crash? Or just hang?
If you're getting stuck at the deletion or library creation phase there is a "sometimes" issue where the library manager will spawn dialogs requiring user input
underneath the main window or windows that show progress. So it looks as though MC is still frozen in process and the process never ends, when in fact there's a modal dialog box asking you to confirm something hidden under the progress box or main window. If this is the issue you're seeing you can typically "find" the correct box by alt-tabbing or using another method of window switching to foreground the hidden modal dialog. It's a longstanding bug that is less common than it used to be, but I still see it occasionally.
If you're seeing something else, please describe more specifically what exact steps you're taking (specific menu commands) and exactly what the results are and we can brainstorm.
I was able to navigate to the Nas Music folder through Media Center though.
Can you explain how you mean? Do you mean in the views? In drives and devices? In the import dialog? One thing I've never been clear on is where your NAS is actually mounted in the file system; my guess is that MC doesn't know where it's mounted either which is a problem for a database-based player like MC. So it would be good to know where you're seeing it and how you're navigating to it.