1) urea isn't harmful, in reasonable quantities. if it was, Users of Pfertz, RootMedic, and other products would have a hell of a problem. Plants use ammonia very quickly. Much more quickly than nitrite or nitrate.
2) Urea, is (NH2)2CO. Ammonium Nitrate is NH4NO3
3) The content is not the significant part. The membrane. I have no idea what miracle gro uses.