Re your last question the line should be:-
"Were all the Grahams here in this court"
as it is part of the sequence where his rather trumped up trial is progressing in spite of his many lady admirers offering the judge attractive bribes to buy his freedom.
The Grahams are the clan of warriors of whom Hughie is a leading member. ( He is more commonly referred to as Hughie the Graham)
The fifteen feet that he jumps is probably down to his anger at being falsely accused and so quickly sentenced.
Finally I would suggest that you try to find a version of the ballad performed by Ewan MacColl who makes Hughie sound like the ferocious brigand that he was as opposed to the apologetic,shambling figure as he energes from Ms Tabor's version.