the mysterious gift

The reward is not based on the required level, but the item level of the vendored Unique items. The item level is determined by the monster that drops it; equal to area level for white monsters/containers, area level +1 for Magic monsters, alvl +2 for Rare/Unique monsters.
You can see the item level by hovering over an item, and pressing the Highlight Items button. By default, this is bound to Alt.

It won't just disappear when you go to Cruel difficulty, so unless you *really* want the Prophecy slot, you can just wait until you have more Uniques you really don't care about.

Yep, you'll just keep it.


Energy From Within is a really good Jewel; definitely a keeper, even if the roll isn't super (8-12%).

Araku Tiki has a Fated version, Ngamahu Tiki, which grants a big chunk of Increased Fire Damage. Your Araku Tiki roll is a bit on the low end though, so if you get the upgrade Prophecy I'd personally purchase a better roll on poe.trade. If you don't want to trade, or don't care much for the roll, then keep it in case you get the upgrade Prophecy.
Aside from having a neat Fated Version, nothing special.

The Ezomyte Peak also has a Fated version, but I personally couldn't care less about it. It's unknown whether the Fated Uniques are accessible once the Prophecy League ends, so it might be worth getting a Fated Peak for the novelty value. Other than that, toss it into a Mysterious Gift.

I'm fond of Asenath's Mark, I like the speed bonuses, but it's none too special. Common, cheap, whatever. Can be tossed into a Mysterious Gift no problem.

Daresso's Passion and Storm Prison are fine leveling Uniques for what they are. However, they also fall under 'common, cheap, whatever' :p If you don't plan on using them, perfectly fine candidates for a Mysterious Gift.