So, Atronachs. What are they? Where are they from (specifically, where in Oblivion)? Why are they found in the service of so many Princes? Why are they so more easily summoned (or so it seems) after the events of TES:IV than other Daedra?

Discuss.

This page comes from the UESP and doesn't cite its sources, so take it with a grain of salt, but I think it's a good reference for the nature of the atronaches.

http://uesp.net/wiki/Lore:Elemental_Daedra

As for your fourth question, it could be possible that Martin's sacrifice placed stricter restrictions on the Princes and their minions, and as independent spirits the atronaches are more receptive to a conjurer's summons. That's just me spitballing though

If memory serves me correctly, they are bits of creatia/aurbic spirits that coalesce in Oblivion into various forms. Being such, they have no necessary alignment to any certain prince (that is, they are not 'part' of any Prince as is the norm for lesser daedra ala Shezzar's Song). As for why they're more easily summoned after TES:IV, mayhaps because upon the destruction of so many [daedric] towers and so much mythic mayham there is more creatia floating around for them to form from, making them if nothing else at least more numerous.

I know i've read somewhere that they often hop from realm to realm as bits of creatia leaking from the aetherus hole (magnus) blossom into outer realms with limited shelf lives, making it a perfect area for them. I can't remember the source however - I feel like it was in kagrenacs writings, or maybe something MK wrote...not sure, but i know i read it either here or TIL.

While called "Frost Daedra" http://www.imperial-library.info/content/shade-perilous could be of Help. Also, I'm pretty sure that Bethesda as retconned the elemental daedra into atronatches, even though the two were separate pre-Morrowind

The book Spirit of the Daedra mentions them for a bit I believe, calling them the Vermai*, so I guess at least some of them are Daedric in nature?

*(not sure, may actually be referring to Clanfears)

Vermai were a type of Daedra in Battlespire.

Anyway I guess its because Atronachs are the most independent type of Daedra that they are summoned the most. Others are usually only summoned by followers of certain Daedra, for example a Molag Bal worshipper would most likely summon a Daedroth. Azura Worshippers summon Winged Twilights etc.