I am kind of surprised at the lack of product information about MC on the JRiver site - very little useful information, so I'm turning to the forum, here.
I mostly agree. Though it's hard to suggest what they could put there instead. All I know is, what is on the site now doesn't really show off a lot of the great stuff about MC. On to your questions:
While I am interested in sound quality and cross-device integration, my primary desire is a tool with strong management/curation capabilities.
MC is unmatched in this capability as far as I know. It's the essence of a power tool for Library Management.
(1) One of the main reasons for sticking with an older version of iTunes is that I ONLY want t music player - I don't want my library manager messing around with videos or TV or images. Will MC cause me any problems because it "wants" to manage videos that happen to be on my computer?
MC will only import what you tell it to. You do imports by directory. For each directory you can tell it what file types to import and which ones to exclude. "Auto Import" can monitor directories and import files as they are added or changed. It is configured the same way and can import only the types you specify.
Further, you can put MC in "audio only mode" and it won't show you options for video or images.
When you first launch MC it will start a countdown and then try to find your Media and import it for you automatically. If you don't want that, cancel the countdown and do your imports when and how you want to.
(2) How can I test MC out without "letting it loose" on my music library? Does it do anything to the music files (e.g., updating metadata)? (I don't want to mess up my currently library files until I've decided to make the switch to MC.)
MC will only write tags to files if you change them in MC. If you change a song title from "Tangled Up In Blue" to "Wrestling with Red", it will change the Name tag on just that file. It will also write play count and that kind of thing to the files. It's really quite safe. But if you want to TOTALLY prevent that while testing you can. Uncheck this option:
Tools > Options > General > Importing & Tagging > Update tags when file info changes
(3) I see that I can add my own tags. I like this a lot. Is there a limit to the number of fields I can add?
Not as far as I know. I've added about a dozen fields to MC's Library database so far. There are a LOT of fields defined already. I imagine there's some really big limit like 16,000, but I'm totally guessing. For practical purposes I think there's no limit.
(4) From what I can tell reading around this forum, it looks as if I'll need to recreate my iTunes smartplaylists. Tedious, but not problem. Anything else I need to know about smart playlists?
MC's SmartLists are very sophisticated and can do some really neat and crazy stuff. Check out some of the examples here. Make sure to look at the ones that use "mix" and "limit":
http://wiki.jriver.com/index.php/Search_Language#Examples(5) How much data can MC "suck out of" an iTunes library (i.e., the library file, the XML file, and the music files themselves)?
You'll want to export your itunes library to MC. To do this, create a playlist in itunes that just has EVERYTHING in it. Then save that playlist in itunes as an XML file. Then import that into MC (File > Import Playlist). This will transfer (as far as I know) star ratings, play counts, and all of the basic metadata (album, artist, etc). I'm not sure about Composer and other lesser used tags. This process will also tell MC where all of those files are, so that should be all you need to do to import all of those files into MC.
Syncing with itunes is a big scary (to me) topic. I know very little about it and I hope you won't have to do it. But you can if you have to.
I think you're going to find that MC will give you enormously more power to view, search, and sort your collection than you're used to with itunes. If you need help figuring out any of that stuff (like building custom views), start new forum topics about it and we'll try to help you.
Enjoy!
Brian.