1. How do I move all contents of my Nice Stuff gallery to the Show gallery?
Charlie, only with trickery I would not even be able to accomplish myself.
So say it can't.
2. Also, how do I delete a duplicate item within a gallery?
Just delete it. Notice that ubnder RighClick on the Library Item, there's a "More Gallery functions" option near the bottom in the context menu, and there reside the functions you seek for.
The situation for 2 is that if I like a track I hit Alt-c. But it turns out I do that multiple times over the course of years because I can't remember whether or not I've hit alt-c before. So I end up with unwanted duplicates.
No, that is not how it works (or indeed you'll get mad). So I am afraid you already "manipulated" outside of XXHighEnd's functions, like copying Gallery folders or renaming them etc. etc. via Explorer.
Say that it is another subject how to get this right again. This too will be quite impossible, depending on the damaga done. Anyway :
When you use one of the "Put to Gallery" gallery functions, it creates meta data of the album in the Gallery Folder with the same name (but prefixed with the Gallery itself). Do this again, and it just does it again. No duplicate can emerge. However,
copy the Original Location (folder) to elsewhere, play that and again use Alt-c etc. and you have the Gallery entry twice. Well, not really, because you have the Original also twice. So this is still consistent.
Now
Rename the Original with Explorer to another name and things will go bad on you; you now renamed the Original but did not rename the Gallery, which you btw can not even do because there's also the metadata itself with related contents). Anyway, if you now plat from this newly named (location) and use Alt-c again, you indeed have two entries, but the old one is actually obsolete.
Do the problem really is that after the rename, the old one is a floating entry with reference to something which does not exist any more.
This is your base for understanding; it solves nothing (but if you have concrete questions, shoot them).
Peter