Actually I did use the logic given in above link.
ie. changed field permission level for (item amount and some other fields) from 0 to 1.
However, the users got the above Missing fields message on trying to save or submit on changing quantity in the delivery note. It saves perfectly when quantity is not changed though.
It is behaving as if fields changed to 1 do not exist when changed by stock user role (which has level 0 permissions only). I thought these roles should only be hidden from the role while automatically picking prices etc. as per rules in the background.