Many good questions, Jack, and I certainly feel the pain of dealing with OneCare backup and restore. I'll answer the best that I can.
Absolutely. The restore process allows you to search, but in your scenario, you will have difficulty identifying the specific files(s) you want by location.
No, you should not have any issues using the restore process to access older backups. However, when the PC goes through a rename or OS reinstall, it is a new PC to OneCare. You will need to use the custom restore option. In some cases, you may need to perform a small backup to the destination from the "new" PC for it to be able to restore from that location. Yes, that would be a glitch, in my opinion.
You can open the backup files with a utility such as Winzip and extract the files manually. Any split files will need to be rejoined. A free utility for that step can be found here:
http://www.freebyte.com/hjsplit/
Is it easier? No.
I am ever hopeful that backup is greatly improved moving forward. It is clearly a pain point in the product right now.
I am not aware of any documentation for this and suspect that none exists. However, based on the fact that the backups are simple .zip files, one could presumably write an app to read the contents of all of the zip files and produce a searchable index from which actions could be executed to extract and rejoin and restore files. Personally, though, I'd switch backup processes (note that I personally do not use OneCare backup. I use SyncToy to maintain copies of my data on various machines and use Windows Home Server to perform a daily image backup of machines.) before trying to develop a workaround for the OneCare backup shortcomings.
Thanks very much for the feedback and I remain ever hopeful that we see some major changes to backup in the future. Who knows - I may even use it then.
-steve