Salamun Alaikum,
In my humble view, it is clear that Prophet Jesus was not referring to a separate
'Book' as a mere child when confronting his people when they parted with allegations against his mother Mary (19:28).
I feel strongly that the mention of the
'Kitab' in this context is a reference to the laws of the Torah, or at least what scriptures were being read by his immediate community. Furthermore, his reference as a 'Prophet' clearly shows the authority granted to him by God to provide wisdom with regards these scriptures which were part of what was granted to him (Injeel). This I have covered in the article you mention.
In 7th century Arabia, the Injeel would have represented the Christian literature being read at the time of Prophet Muhammad's (pbuh) ministry. This would have most likely consisted of a set of 'teachings' based on the wisdom imparted by Prophet Jesus (pbuh) and would have comprised of canonical as well as some non-canonical literature / beliefs.
With regards verse 7:157, your inference is correct. It is my humble view, that there must have been a
'coexisting' scripture which the contemporaries of Prophet Muhammad referred to as the 'Injeel'. Otherwise, there would be no point in referring them to a Torah or Injeel to ratify a claim.
I have a dedicated article which elaborates further on verse 7:157. Please kindly see below.
IS THE NAME 'AHMAD' FOUND IN THE BIBLE?http://quransmessage.com/articles/ahmad%20FM3.htmI hope that helps, God willing.
Regards,
Joseph.