I just read http://www.imperial-library.info/fsg/nalionarticle2.shtml and I must say I'm very impressed. I'd like to discuss however the events in SI in light of this theory.

1. The "peaceful gate" Sheogorath opened "brakes no covenant" which doesn't seem to fit in with the guardian theory (wouldn't it be required for Him to ask permission from the guardians and/or "not break the new covenant"?)
2. How do you relate the Fountain of Madness to a tower? I think it might be more anological than meets the eye (but with different source of power)
3. Based on #2, what do you think about Sheogorath as a "guardian" of the "tower of madness"?
4. How do Sheogorath's past actions come into play? Consider his interactions with ALMSIVI (both in-game lore and the famous trial) etc.

More to come...

1. There always were ways to circumvent the barrier, there are two more well-known places from which transport to Oblivion (space) is/was possible: the Dreaming Cavern of Artaeum and the temple which provided transport to the Battlespire. Also certain powerful individuals were able to summon even Daedric Princes. The barrier never was an absolute barrier, there were ways around it and I don't think that invalidates the Guardian Theory as a whole. The barrier acts as defense and as such, there may be no need to defend against a gate like that to the Shivering Isles.
As to the Sheogorathian Gate: to me, it is primarily an invitation and it happens in a time where the status of the barrier is uncertain (been some time since I played Oblivion nad SI, isn't the gate open right even at the start of the game?).
Also, a side thought: I think need is a key factor in many events in the TES world, the need of a hero to fix things. Also, the events of the Shivering Isles are part of a prophecy (Grey March etc.). "Each event is preceded by prophecy, but without the hero there is no event."
So: If a prophecy can allow Sheogorath to drop burning dogs from the sky (K'sharra prophecy), then, yes, I think that the need for a hero and the word in form of a prophecy is enough to allow something like a gate to the Shivering Isles for the hero to enter.

2. and 3. Played through the SI too quickly (and was not very convinced by it) to properly answer these questions.

4. I'm not sure where you aim with this question, care to elaborate?

1. Leaving Mundus has never been prohibited for anybody but Lorkhan[1][2]. It isn't easy but if simply willing to be there is enough[3], it can't be obstructed. It's the Daedra who have problems getting in. They need people on the side to break through the barrier and let them in[4].


[1] Words of the Clan Mother. The secrete Y'ffre shouted across the skies.
[2] The Monomyth. Lorkhan is trapped in Mundus.
[3] Doors to Oblivion.
[4] Liminal Bridges.