This one I have fixed and pull request is incoming, just opening this to link it to it.

Pull request has been submitted, but to clarify, this is what it's doing

https://i.imgur.com/zJIqm9s.png

(there may be more needed to get it to work with the enchant illusion slots as well, but I'm not familiar enough with the xmog code to address that part)

I think I may see an issue with slot IDs and whatnot in general.

Does this command if run in game produce the output you're expecting when you loop 1-19?

/run for i=1,20 do local x=TransmogUtil.GetSlotName(i) or '-'; print(i..': '..x); end

EDIT:

Output of the command so you don't have to login

1: HEADSLOT
2: -
3: SHOULDERSLOT
4: SHIRTSLOT
5: CHESTSLOT
6: WAISTSLOT
7: LEGSSLOT
8: FEETSLOT
9: WRISTSLOT
10: HANDSSLOT
11: -
12: -
13: -
14: -
15: BACKSLOT
16: MAINHANDSLOT
17: SECONDARYHANDSLOT
18: -
19: TABARDSLOT
20: -  

EDIT2: Ignore that I went to 20, that was just me making sure it'd handle nil correctly in a oneliner

Added provided updates.