You are right about the two clues where there is an ambiguity about which letter to add. I would suggest that you take a close look at 28 across and ask yourself if you've really solved it. It might help to use the names index in previous editions of Chambers.
I also suggest making a full list of the number of times you've used each letter in the two types of amendments, and see if that gives you any ideas.
Hope this helps a bit.