1. Because it is a generic trope which appeals to casual fantasy players.

2. There are many more et'Ada beyond that which mortals classify as the Daedra, Aedra and Magna Ge. Potentially infinitely many, as Oblivion is infinitely large and contains "all possibility".

To encompass or number the et'Ada is a futile endeavor, for they are beyond mortal comprehension, and our perceptions of these beings are but faulty shadows of their complex reality. Many a savant has been driven mad by the attempt.

Your problem, mortal, is exemplified by your words, 'share a common origin in the planes of Oblivion.' There is nothing 'common' about, between, or across the planes of Oblivion—they are the very definition of change and variation, manifesting all possibilities, and validating all understanding and misunderstanding. You seek similarities where there are only differences, a classification of chaos.

1. Well, time and causality do not apply beyond Mundus. Trying to attribute a fixed power level to acausal beings isn't going to get us an answer.

But we do see the power of the Celestial Guardians in ESO. I'm unsure whether they are actually Magna Ge, I think they are implied to be. The Serpent Celestial was going to break open Nirn's surface and engulf everything in magma, destroying all civilisation. The Warrior Celestial claims to have a more powerful Shehai than the first-ranking Ansei who sank Yokuda. The Mage Celestial lost control of her power temporarily and blasted another plane into existence, and her unstable "staff" was going to turn Craglorn into a massive crater.

The answer of whether they could, in theory, stop the Planemeld is simply going to be yes. If any cataclysmic events like the Planemeld actually occurred then BGS would have no further games to make, hence, power is usually dependent on the plot. It doesn't mean the Magna Ge are more powerful than the Princes, but they would win, because they need to for plot reasons. You see this a lot with the Aedra.

1. Perhaps they might be able to if Mundus becomes undone, but I don't think we know anything for certain.

2. I don't think that is right. Usually the Princes' spheres are far larger than mortals know or can even comprehend. They do not necessarily encompass concepts created by mortals. Either some of the Princes' concepts pre-dated the Mundus, while others changed parts of their spheres to encompass things in Mundus. But even in your example, hunting is not something exclusive to Mundus.