I have a half gallon jug of juice I am about to mix and use. Is there a problem with pouring the excess juice back into the same jug it came out of? If not can I use an empty distilled water gallon jug.

While you certainly can pour it back into the original jug, I recommend never mixing used and virgin Cactus Juice except as needed to fill the chamber.  The reason is if you contaminate the Juice in your chamber, say by doing an oily wood, when you pour that Juice back in with virgin Juice, you may ruin it all.  Another scenario that has happened with a customer is their power went off while the vac pump was running.  This caused the vacuum in the chamber to suck the oil from the pump and dump it into the chamber.  He did not realize it and when he was one stabilizing, he poured his excess Juice back into the container with fresh Juice.  The oil from the vacuum pump contaminated the entire batch and it would not cure.

You can store you used Juice in any container that is not air tight.  My preference is the quart paint mixing cups with the plastic snap on lids you can get at any good hardware store.  Your distilled water jug will be fine too.