The Psijic Order looks out for themselves, just like any organization. They may claim that they are guardians of Tamriel, but their motive with the Eye of Magnus is clear - to take it for themselves. I suspect that the only reason that they did not take it until the end of the quest line is because they divined the future and saw you and Ancano fighting over it - to prevent that from happening would have created a temporal paradox. That's speculation, but it's clear that they can divine the future in some way, and the Elder Scrolls establish that history is filled with 'set' events that cannot really be altered (though the Thieves' Guild line in Oblivion discredits that theory somewhat).
The Slow Time shout in Skyrim has a much smaller window of opportunity, slows down the user, and does not if fact stop time as simply slow it down to between 30 and 10% of its original speed. What the Psijics do is stop time for prolonged periods, with a complete effect on anyone other than the user and specific individuals they do not wish to slow down, and it remains at full capacity for at a pure minimum two minutes - how long the conversations can take. Of course, by not following through and asking questions, or by moving away from Quaranir, it is possible to extend the time stop effect into the tens of minutes - 20 times longer than the strongest Slow Time.
If Stop Time made you ethereal, surely you would change appearance as per the Become Ethereal shout, but you do not. You look exactly the same, as does Quaranir. The color-bleeding effect is a side-effect of the Time Stop spell, as seen by the Slow TIme shout. In addition, I'm fairly certain you can still pick up objects while in Stop Time, for example when in the Archmage's room.
None of that changes how they can teleport not only across large distances (hundreds of feet at least), they can teleport extremely large objects with them, and they can do so straight through physical objects (they do not need a line of sight to teleport). At the very least this ensures that no-one who ever has to sleep is safe from them, even in the most heavily guarded fortress.
Ultimately many of your points are speculations at best, while I'm trying to analyze the exact nature of their abilities by what is presented to us in-game. Comparisons with Dishonored, while interesting, are not useful as the Outsider's magic is entirely different from Tamrielic spellcasting.
Yes. I actually support that he is in Akavir. I was just astonished with the guy who didn't know about the Nerevarine.
I actually wrote something related to that in the What kind of boss do you want for TES 6 discussion.
Game balance does not always equate with Lore balance, and frequently does not. Mehrunes Dagon is not balanced with standard humanoid creatures - he's a bloody Daedric Lord. The Psijics are not necessarily balanced with standard mages, in fact Lore states they are much, much more powerful.
The Psijics can completely stop time for no less than a full minute, and teleport at a base minimum 200 meters (likely much, much farther given how far Skyrim is from the Summerset Isles). This is enough to elevate any one to "powerful enough to kill anything other than itself". At the very least, one could reasonably stop time, grab you, teleport you to ten miles above Red Mountain, let go, teleport back, and start time again. If that does not kill anything less than a Daedra or Aedra, nothing will.
Dishonored has nothing to do with the situation - the Psijics are clearly on a whole new level of power compared to your average mage. The only thing you can hope is that someone Watches the Watchmen.