The Continuity Gem has only appeared once in Deadpool Vol. 3 Issue 27, and even then only in the "Continuity Spontinuity" storyline. In it Deadpool finds the Gem which has the power to retcon Marvel comics history and also talk to the writers apparently.
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Deadpool throws the Gem away at the end of the storyline and it hasn't been ...
No, there is no evidence that the Ego Gem is related to Ego. The Ego Gem is the main consciousness of the Infinity Gems and when reunited with the other 6, it manifests as Nemesis a being capable of creating universes.
Ultraforce/Avengers Vol. 1 Issue 1 pages - click images to enlarge.
In fact as Ego, as far as I'm aware, is a being in a universe it ...
Because Earth i.e, Midgard is located at the center of the nine realms
From MCU's perspective
As Thor describes in Thor: The Dark World,
Thor: Well Heimdall had lost sight of you, you were no longer on
Jane Foster: Well, how's that possible?
Thor: I believe you were in between worlds. The Nine Realms travel within Yggdrasil, ...
It is difficult to explain the importance of Earth without using the out-of-universe explanations that you've already given.
In terms of why the events of the War of the Infinity Stones takes place on Earth is both simple and complicated at the same time; it happened on Earth because most of the combatants had a tie to it, and that it was a gathering place ...