Don't worry about Diacetyl.
Popcorn lung effected literally a handful of workers in a popcorn factory, and it wasn't even proven the diacetyl was the cause.
EDIT: That isn't me in the video BTW, just a video I thought may ease your mind. Also, when you smoked there was a lot more diacetyl in those, and not one case of so called popcorn lung, or bronchiolitis obliterans, has been diagnosed in any smoker ever.
Mmmm...popcorn...Salty or Sweet tho?
V1 Send it to me, send it to me.