Adding items is experimental and will not work for most things, please a bit of patience, the reason why you are having issues is outlined below.
You can use it for seasonal, but adding legendary gems doesn't work well at the moment.
Blizzard IDs are as they show in the editor DB, this is exactly correct and why add item doesn't work for most things, add item was added as a quick way for me to test item IDs, I added it in 2 minutes just to fit a need I had, instead of keeping it to myself I decided to make it public in case it's somewhat useful to you guys. It will get expanded to be usable in early january (after christmas)
This is not possible, the reason being that affixes are actually hardcoded on the game, when an affix is 4.5-6% critical hit chance that's what it is, you cannot add to the value because the value is just defined by a seed that determines where in the 4.5-6% range the affix will roll. Blizzard seemingly did this because in D2 it was easy to jump in and add retarded numbers everywhere so they learned their lesson.
Chances are you are not loading the modded save, or not transferring it back on the switch on the right folder, these are just guesses... The editor isn't perfect but something as basic as saving values works
TL;DR: Add item is experimental, I say it everywhere, everytime, so just wait until I actually do something with it (sorry!).
I'm on holidays in my home country so I will not do much until end of year.
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Oh, did you get it from the .gam files? I couldn't really get my head around extracting those and parsing them if there was a better way I'd like to know (for science).
I will add these items to the DB as soon as I can.