I can answer the second part of your question. Since you just updated the MacBook's iTunes settings to point to the NAS, it's iTunes library file still resides on the MacBook. This is a .itl file in your ~/Music/iTunes folder.
iTunes relies on this database to tell it what's in the media folder. If you add stuff to the media folder without adding it through iTunes (i.e. just dropping files in Finder, or using another copy of iTunes on another machine) then that .itl database does not get updated.
So when you add files through iTunes on your Mac Pro, it updates the .itl database on your Mac Pro, but not your MacBook, so the MB can't see those files. If you add anything through iTunes on the MB, the Mac Pro database won't have it, since the copy of iTunes that connects to that database did not process the files.
I have a similar setup to what you're doing: iTunes media on an external drive connected to my iMac, and a MacBook which I use ONLY Home Sharing on.
You might get the idea to move one .itl file to the NAS and open it in iTunes on both machines, and this will work, but you can/should never have it open in both at the same time. That .itl file is really just an SQLite database, and they don't allow simultaneous access, so I've never even attempted this.
With respect to your first question, check ~/Music/iTunes and see if there are any old library (.itl) files. It's most likely that iTunes is opening an old copy or a backup of the database. If you duplicated the database or started a new one before moving your media, this might be the cause.
Start iTunes and make sure the media folder is set correctly or make some other change in iTunes. Now go to ~/Music/iTunes and look for the .itl file that has most recently been modified. Move all the others out of the folder.
Quit iTunes and restart it while holding the Option/Alt key. This will bring up a dialog that lets you select which database to use. Select the .itl file you left alone and it will continue using that as the default library.
What you need is a network location manager program, typically these allow you to perform certain actions (e.g. connect to a network drive) when the machine is on some networks and not on others. Several years ago I was using one, now defunct, so I can't recommend any from experience. What I found now are things like Sidekick ($29), Control Plane (free).
You can also do this with a script (e.g. Login Item, if you log off or shutdown the machine when moving it from home to work or vice versa, or cron job) to determine the network the machine is on and depending on it to connect to the network drive (if not already connected) or not.
Best Answer
First you'll need to make sure that the network volume you want to use is mounted first. Then you should be able to just navigate to whatever folder you're planning on using as your new home directory and select it. Network volumes in the OS X Finder can typically be found in the "Network" item under the main directory of your computer (in the Finder, you can hit ⇧⌘K to go there directly). The absolute path if you're using the terminal is typically /Volumes/Share Name.
It's worth noting that if you login without the network drive mounted, things could easily break. For this reason you'll want to make sure that you've set up OSX to mount the share on boot (prior to login). This doesn't happen with SMB (Windows file sharing) or AFP automatically, but there may be a way to hack it, maybe others can chime in with this part. NFS shares set up through Disk Utility mount automatically, but I'm not certain when in the boot process they do, I believe it's pre-login, but I can't find any information to corroborate this.