"You have me worried now! When you say it will "clone over" does that mean that any pre-existing documents (in the Documents folders) will be deleted?"
In a regular clone, YES.
The files that are currently there will be REPLACED BY the files getting "cloned over" from the external drive. Unless some of them are the same files, in which case they'll get updated or be left alone.
I believe CCC has an option to (and this may not be the exact language) "preserve newer files, don't delete anything". With this option, files on the source will be copied to the target, but the files pre-existing on the target will be "left where they are".
I think this choice would work for you. It would merge the contents of both documents folders.
"The External HDD does not have a an option "ignore ownership on this volume". I've seen screenshots of this but for some reason "get info" properties does not present this option?"
Look again.
This time, under "sharing and permissions".
There may be a "disclosure triangle" you have to click to see the options.
Personally, I have never encountered a drive or volume that prevented me from doing this.
(That doesn't mean they don't exist).
"As for the 300gb in one folder? Surely, OS X can handle this?"
I will say again (and it could be nothing more than an instance of my own prejudices and superstitions) that one shouldn't keep that much data "in a single folder".
What the heck do you have in there that constitutes 300gb, anyway?
If there is a mix of movies, music, pics, other files -- I would break them up and spread them around some.
BTW, I don't use CrashPlan or waste my time with ANY "online backup".
I maintain numerous physical backups and keep them dispersed in different locations for safety.