Spoiler Okay, to clarify, Daedric Princes (aka, Mehrunes Dagon), are inmortal and cannot be destroyed, but only banished into their realm of Oblivion.
Actually where told that all deadra, (and presumebly even Deadric Princes are subject to this, i can't remeber weather there's an explicit statment on the matter though), when killed will be banished to a sort of ether realm out beyond even oblivion where they must return from in "spirit" form before they can go back to their normal buisness.
Spoiler Thus presumebly Dagon was banished there and until he can return to his plane, (or possibbly planes, some deadra have many of them), there is nothing powering the gates, effectivlly shutting them down
Regarding the Sheogorath's portal my only point was that if the barriers where back up that portal is impossibble. The nature of it is irrelevent to weather it';s possibble or not.
@Prowler: I suspect your simply representing your data in a manner i find confusing but several of your statmebnts don't track with various Oblivion sources.
To go through it as a rough list thing:
A) The Barrier is just what it says. Nothing can get through without expenditure of power on one side or the other. Deadra for some reason are easier than Humans.
B) Any kind of deadric summoning is excepinolly difficult even with large amounts of power, and permanantly binding a deadra to this plane requires still more and a great many complex rituals. Why syuch power is needed to keep them here permanantly seems top vary with the authour. Some state the deadrea need a supply of magic to live here, other say they need a prison of magic to stop them fleeing back to oblivion and some seem to imply the barrier itself constantly trys to pull them back. It's never really clarified.
C) Permanant stable portals bewteen mundas and Oblivion are impossibble. As are long term ones. One of Oblivions books describes how to create a short term one, (it bassiclly involves crafting a sigil stone on both sides and the deadric prince pouring a lot of his/her power into it). However whilst the resultant portal allows free passage from Mundas to Oblivion and back again, (and it's the only known way according to the book), it will last just minutes before the sigil stone burns out and the portal collapses. When the barriers came down not only as Dagon able to create permanant portals, but equally he required far less preperations, (hours instead of weeks) and no rituals on the Mundas side.
D) Since the Barriers where put up no deadrea or deadric prince can pass freely between mundas and Oblivion. They can come here if summoned just like abny other deadra, as Dagon does at the end of the first age, and as various other deadra are stated to have done throughout history. But they can't just wll themselves here. Thats one of the major points of the barrier. Martin Also backs this up stating that Dagons apperance is evidance the barriers have failed completly, implying that he cannot normally appear at will on mundas while they remain even partially up.
E) in 2920 some of the deadric princes agreed to furthar restrictions. This bit has never been clear to me as they didn't agree to much if anything that i saw that they whern't allready prohibited from doing. As somone else has pointed out, Akhotosh and Sotha Sil are sepreate entities so there's nothing binding about it and it can't remove any of Akhotosh's restrictions.
F) Whilst we get a number of cases where the act of sumoning a deadra Lord is not explicitly stated, (meaning they might not have been summoned, but then again they might have been) the only Deadra Lord ever explicitly stated to appear without a prior summoning ritual in the post alessia eras is Azura. And she just seems to break rules left right and centre really. The problem of course is she is the most stufdied of the deadric princes and was written in detail well before oblivion when the barriers thing just wn't pat of cannon. It's a case of writers error, but for an in-universe explanation i'm stumped.
I think that covers the siuation as it's laid out in oblivion, (which itself seem to differ a littile from past interations where the difficulty of interactions was majorly toned down).
p.s. why did you mention Loken? Loken had been dead and bueriued for an age or more by the time the barrier went up, so weather it keeps him out or not dosen't really matter, there's nothing left of him to keep out by this point.