and it's an extra step that will really slow them down
You can setup a custom subject for every client like this
Client A - %JobResult% - %ComputerName%
Client B - %JobResult% - %ComputerName%
Client C - %JobResult% - %ComputerName%
I have read about the backup cache, but have not tested it. Will it help with the merge?
Not really. It allows to create incremental restore point locally when remote target is offline. When the connection is up, special process starts to upload the cached incremental backup to the remote repository.
Merge might be slow if you have long backup chains. In such case, periodic active full backups may help