If they use Steam as DRM then yes, Steam is required.
Which, also, was never stated. All it said was it was using Steam instead of GFWL.. nothing more. Dark Messiah uses Steam, but you can choose not to install it or ever touch it (which is precisely what I did).
They didn't make a PC UI for the last two games, why would they now?
Because the last two games were essentially the same engine using the same basic UI. This time around, they're taking a serious look at improving the UI.
And yes, "dynamic" means scaled to your character with randomized rewards.
No it does not. Not according to the information we have, they say nothing of scaled quests, just quests that can be changed according to how you've played, where you've been, etc. They say there's FO3-like area level scaling (which, BTW, I'm not very fond of), but this is not as bad as Oblivion. And also, randomized != scaled.