We know we will visit the Soul Cairn as a means for a quest. And there will be some side-quests too. But will it be like Sovngarde where we go once and never return unless we restart or reload?

From the interviews, videos and screenshots, we know that the Soul Cairn will be large in size with fortresses and many castles. So can we visit there even after we completed the quests there?

MAybe, there's really no way to tell. They'll have to give a good reason to, becasue if they just left a portal there to Oblivion people would start to get teh wrong idea.

I think you can visit Soul Cairn as many time as you want because Todd Howard said that you're able to summon that new mount, Arvak, anytime you like from there. Unless there's like a spell to do it.

An area that large with many buildings and side-quests gives us reason to believe we can visit and revisit the Soul Cairn. A smaller area than the Shivering Isles, but with a possible gate in Castle Volkihars' basemant.

I was curious if we had any confirmation.

But didn't they have the portal to soul carin in the castle.Plus if they have all this stuff in there they cant expect you to expore it all in only one mission there. so I think it would be like shivering isles where the portal is always open

Well unless you destroy the portal or its power source i see no reason why it will close.

If the only way is in Castle Volkihar, will Dawnguard members be able to enter it?

If they raid the castle then they may have to kill vampires which are in the soul cairn. I don't know though.

Maybe it will also be in dawnguard castle but nobody will know untill the dlc comes out

I imagine the Dawnguard tunnelling underneath Castle Volkihar to reach the portal to oblivion.

Others will probably not imagine that, but I do.