A video or step-by-step guide for cleaning the barrel without damaging the hop-up rubber?

The best thing you can do is disassemble the inner barrel and clean it well inside with a cloth with isopropyl alcohol, but I understand it can be scary to disassemble part of the replica, so I recommend you take the cleaning rod, place it next to the barrel and put the tip of the rod where the hop-up would start, push it back a bit more and put your fingers where your replica's barrel starts, now you know what the rod limit is that you can insert, once you've done this you can mark it with adhesive tape for example and give it a good gentle clean with the cloth inserted in the rod.

In any case, I'll leave you a video so you have a good day, Irene.