I have heard many pronunciations of the word सुबह, such as su - ba - ha, su - b - ha, su - be - he, su - bai, su - baa, and even su - bhaa. Which of these is the correct pronunciaton in Standard Hindi and Standard Urdu?
sub'h is just a literal transliteration. Most likely the pronunciation goes like sub'ha. The a after h is a short a vowel, i.e. the schwa.It's sub'h in standard Urdu, and that is how it is used in Urdu poetry, although in colloquial speech 'subah' is a more common variant.
No, that is not true. The word is indeed a monosyllabic word in Standard Urdu and that is how it is treated in Urdu poetic tradition, whose meter is measured by long and short syllables.sub'h is just a literal transliteration. Most likely the pronunciation goes like sub'ha.