a - Yes, you would have to Uninstall and reinstall the client using only the components required
b- You can look at the Installation and Licensing Guide, or the particular Applications Integration Guide, but, to be very honest, this information is kind of hard to come by. Generally speaking, if there is an Applications Integration Guide, then it requires an OnLine extension license
c- Since VSS backup is now the default for Windows File system backup, you don't need an Online extension license for your basic Disk Agent Backups. However, when you start getting into Disk Arrays, and Instant REcovery, this does need to be licensed