You do realize that she doesnt have the responsebility for all of blizzards lore right? That its vetted by a lot of people before it just gets released?
1. No. The helm was said to have been created by the nathrezim which is still very possible.
2. We dont have official confirmation about when Sylvanas started working for the Jailer. This is speculation.
3. Thats not a retcon. You still go to a shadowlands like world when you die on azeroth, outland and so forth and run to your body.
4. Where are you getting this from? Feelcraft.com? The helm was the link between the worlds hence why it shattered the barrier. That doesnt mean the item itself was inherently more powerful to the person who wore it