Is it accessible by just flying up to it from skyrim? (assuming my character can fly)

I believe Skuldafn has its own cell, and activating Odahviing is the only way to my knowledge to load the world of Skuldafn. If you just noclipped or flew or whatever, you'd probably find a bunch of unloaded stuff.

It's not on the map, it has it's own worldspace. Like Japhet's folly.