You can't really edit saves in the way you want with the construction set, but you don't need to. If you load the master file(s) (ie Morrowind.esm) and look in the object window, switch to the clothing tab, then you can find the amulet and take note of the ID - in this case it's "Julielle_Aumines_Amulet".
To check if a trader has it, open the console, click on the trader so the console window is focussed on them (the NPC's ID will appear in the top bar of the console window) and type:
GetItemCount "Julielle_Aumines_Amulet"
then hit return. That will tell you how many they have (ie if it's 0 they don't have it, if it's 1 they've got it and are probably wearing it).
To force them to un-equip it without selling them anything else, you can use this (still with the trader selected):
equip "exquisite_amulet_01"
An alternative way to find who has the amulet would be to use Morrowind Enchanted Editor. It's not so user-friendly for beginners but it can load saves directly and you can search on the ID of the amulet.
PS http://www.fliggerty.com/phpBB3/viewtopic.php?f=53&t=1008 stops merchants equipping things you sell them, and fixes a lot of other bugs/annoyances, including savegame corruption - and you can choose which fixes/features you want when you install.