Can I backup a Time Capsule to an external HD for redundancy?

Yes, you can. Probably the simplest way to do this is connect a USB drive to the USB port on the Time Capsule and use the Archive function in AirPort Utility to make a complete copy of everything that is on the Time Capsule disk.

This is a slow process, so you might want to allow 3-5 hours, on average, for this procedure.

The problem with archive is that it will backup the whole TC every time you do it. Unlike an incremental backup this involves backing up everything you backed up the previous time so take a long long time.. and there is another problem.

If you are backing up your TM backup.. if an error gets into the backup.. not so unusual.. you will be copying the error.

It is better if you want a second backup to simply do that directly to the USB drive.. which can be plugged either into the TC or direct to the computer.

If you are backing up files on the TC you have copied there.. use CCC or another 3rd party backup so it can handle incremental backups but it will be slow.

Can you perform an archive then use CCC to perform an incremential which in theory should be fairly quick depending on the new/edited/deleted files?

No.. do the whole thing using CCC..

Make sure you have ethernet connection from the TC to computer.. (wireless off to speed things up).

You can do the first backup.. with the USB drive plugged into the computer (using a recent USB3 Mac and of course USB3 drive) will give excellent speed. Much much faster than plugged into the USB2 on the TC where the files will need to be copied from the TC to the Mac .. back to the TC.. then across to USB.. Once you do the first backup CCC should be able to do incremental with the USB plugged into the TC.. it will take some time.. at least at first but it should quickly be able to handle this.. CCC is well optimised.. but I have not done this configuration.. so you might need to play around abit.. certainly CCC has the ability to do far more than Time Machine.. I am just not sure if you need to setup as it will be used for incrementals or you can use the above tricks to speed things up.

Also, I forgot to ask if the TC has a trash/recycle bin to retrieve mistakenly deleted files or folders?

No it doesn't.

And if you are running live files from a TC.. we recommend against it.. especially over wireless you have excellent chance to corrupt your files. Apple clearly says for some applications like iphoto or photos not to use network drives.

I think you need to tell us a bit more what you are doing.