In relation to your first question, a defined omnirc file on the cell manager will only affect backups running ON the cell manager. It wont affect another backup disk agent on another host as each host will have its own omnirc.
yes other clients may not even have a omnirc configured or might have omnirc but not have treewalk defined. So those clients wont have the treewalk disabled. Only cell manager based backups
The DP cell manager "global" file is the only config file that would affect the WHOLE cell...