Am I correct in thinking that if Bob_00001 clones 10.6.8 and then runs Time Machine after upgrading to Yosemite he'll be able to save some/all (?) data changes from TM back to Snowy if he reverts?
That is correct, and is one of the many advantages of Time Machine.
You can never have too many backups, so it's a good idea to
also back up (to a different disk) using SuperDuper or CarbonCopy, but at least one of your backups should be Time Machine.
Be sure TM has a current backup, then install OS X 10.10.x Yosemite. Let TM continue backing up to the same disk. (There is some Terminal magic that
may be required to make TM understand that your Yosemite disk is the same disk that it used to be backing up under 10.6.8 Snow Leopard. Alternatively, if the TM backup volume is big enough to hold two sets of data, you can get away with letting it back up Yosemite as a different source volume.)
If you decide you want to revert, first use TM to do a full disk restore from your latest Snow Leopard backup. (When you do a full disk restore, TM will give you a choice which of its many snapshots you want to restore. You
do not need to restore the latest.) Then, use TM again to restore just the files that you updated while on Yosemite.
Note that this second restore is essentially going forward in time. You're restoring files that haven't been create yet! It's a poor time machine that can't travel both ways through time.
