Challenge 1, Question #1 has been answered - The mystery Lieutenant-Colonel is Honorary Lieutenant-Colonel F.B. Brown according to the accession list for the photo. He was Honorary Lieutenant-Colonel of 15th Field Regiment RCA from 1959 to 1961.
This however raises two additional questions:
Question #5 - Why is Honorary Lieutenant-Colonel Brown at a 43 MAA Regiment dinner?
Question #6 - Who is Honorary Lieutenant-Colonel F.B. Brown?
Challenge 2 is a more somber occasion. The question is: When did Lieutenant-Colonel Francis Cassillis (Bud) Garrett pass away?
Additional photographs are displayed on the Who, what, where, when page.