For me it’s just easier to ‘fix’ the metadata in the way Plex wants it before it’s added to the library (best case), but I have never been disappointed while fixing an Artist or Album while it’s in the library (YMMV). Typically adjustments should be made outside the view of Plex.
Let’s take Pink Floyd - The Wall:
Name/Structure:
A Music Library/
…Pink Floyd/
…Pink Floyd - The Wall/ <-------- Plex and Last.fm want to see this - just like this. Resistance is Futile.
…Disc 1/ <--------- Plex wants to see this - just like this. Resistance is Futile.
…tracks
…Disc 2/ <--------- Yea… self explanatory.
…tracks
You can fiddle endlessly, click after mind-numbing click, trying, failing, trying again, failing again, then suddenly stumbling on the right combination and success - or… in 15 seconds load up Mp3Tag, fix the metadata and move on to the next adventure in your life - like listening to Pink Floyd - The Wall, for instance.
Loading Disc 1 in Mp3Tag, clicking in the large window and doing a ‘Ctl-A’ (select all):

Note in the left pane the box: ‘Discnumber - 1/2’.
Load Disc 2 and make it ‘2/2’
Prepare for Comfortably Numb 'cause you’re almost done.
Before you leave Mp3Tag just check your Artist, Album and Album Artist as these are the most important bits, but you can also drag your cover image into the space provided and streamline that process - just in case last.fm can’t find The Wall cover (or you load the CD for your garage band and they’re not listed at last.fm).
That’s about it… click save (that little blue floppy icon, of course).
Drag ‘Pink Floyd - The Wall’ into your ‘Music Library/Pink Floyd’ folder, update the library and watch the magic happen… turn the volume up to 11 and enjoy.
http://www.mp3tag.de/
In a few minutes you can fix all those ‘Various Artists’ titles you have as well as customize any stragglers or misplaced artists or albums.
My music library was a horrific mess and for a long time I just ignored it because I was sure it was going to be more of a PITA than I was willing to invest in it. Mp3Tag changed my life - and my Music Library into something I can enjoy. Basically I’m lazy, old, and nearer to death every minute I live so I want to do something else other than fight with Plex. I just do what it wants, take my pills and enjoy what’s left of… this… 
PS: Tired of constantly riding the volume control 'cause all your stuff has a different gain level? Here ya go… 89db (the default) is the industry standard:
Mp3Gain - http://mp3gain.sourceforge.net/