The analyses of a text #2

I threw myself flat, then rolled toward the shadows to my right. Simultaneous with this, I heard a thunk from the side of the building across the street. The first glance I could spare in that direction showed me an arrow protruding from a wall, its height and position such that had I taken the dive it might well have hit me.
["Trumps of Doom" of The Great Book of Amber by Roger Zelazny]
I'd like to know why it is "simultaneous," not "simultaneously."
And I'd like to know if "being" is implied before "such."
Thank you in advance for your help.