Let's start with No 2 question.
Q. would I key the date of admission for the main individual as the event date for this secondary person also?
Q. it is impossible to know if this relative belongs to the same parish.
A. If the parish is not clear then mark parish field BLANK.
Now Question 1 - and I'm guessing a bit here as I can't quite visualise what you're seeing.
Q. the name of an additional child written above the line
A. Sometimes when I've seen this it is an additional name for the same child; eg Maria Louisa SMITH where the 'Louisa' is written above the line to indicate this should have been included. Do you think it's something like that?