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?
It all depends. If he has used an upgraded app in Yosemite that has features and/or data structures that were not present in the Snowy version then the answer is a definite NO. For applications with changed library formats he would have to rebuild the library and possible re-add the new items to the library. In other words what can and can't be back-fitted will be application dependent.
