and yet the lore contradicts this by allowing for dremora to be conjured.
No, the lore doesn't. Oblivion gates are not the same thing as summoning daedra. Oblivion gates are a specific form of construct created by using a Sigil Stone to punch your way into Oblivion from Mundus. Prior to TES IV, these gates could never be stable and closed within minutes at most. During TES IV, the barriers were weakened so that these gates could exist as stable portals. After TES IV, the status quo is restored, and stable gates cannot exist.
. sheogorath made a few portals to the shivering isles etc etc.
Two-things, the SI portal appeared during the Oblivion Crisis. Secondly, the SI portal did not allow the traffic of Daedra. It was a portal for mortals only. That changes things.
i bet they could be re-opened individually if a powerful enough mage were to step in.
Even if an oblivion gate was "re-opened" it would close shut within minutes.