I'm mostly interested in the word immediately below the date (September 13),
Looking at the page as posted, I think perhaps it's the abbreviated place of residence of the parents:
Coldin: = Coldingham?
Oldham: = Oldhamstocks?
Cock = Cockburnspath?
(Just from looking at a map. However, Coldingham is about 10 miles distant, and someone who knows the area might have a better suggestion.)
the word or symbol before John Lyell's name,
It’s an ornate capital
B for
Baptised.
is the child's name "John Lyell" or "John Laird" (a middle name)
Neither. It’s not a middle name, but
lawll = abbreviation for
lawful (legitimate). Compare other similar entries on the page.
and are the names below the names of witnesses of the christening?
I would say so. That is where you would expect to find the witnesses' names, and they are prefaced by a letter
w.