AFAIK, for the time being, all ROMs that I've used are 'flash from SD card'
the update is stored on the SD card. You can copy from the SD card to the computer. I'm not certain what format the update is, but I can look for ya.
Odin is just used for returning to stock ATM.
CWM and root only needs to be done one time then you can use it as much as you want. It never goes away unless you use Odin to return to stock.
your 'backup' can be 'restored' in any ROM as well (as long as its data restore only)
I'd spend a bit of time looking around for the 'update.zip' that is the stock ROM. This way, if you tire of a custom one, you can apply the update.zip from the SD card and restore data quick fast in a hurry.
Hope this helps
EDIT: The backups are made as follows:
on the SD card in a folder called 'clockworkmod' then in a folder with the title being the date the backup was created.
In the folder you have the following files (disc image files):
cache, data, system AND a NANDroid.md5 to verify that the restore was a good one.
I would imagine that they can be copied from the phone and kept on your computer for safe keeping.