30a is one of the "Scrabble" words. Muscle is the definition. You need a three-letter word which means "twitching" and the first two letters of the last two words, which gives the actual entry and you have a spare letter which goes in the right-hand column.
12d: definition is "medium", first letter is "Durham's" "tail" followed by "a", and a three-letter word for a pain, into which "l" for "length" is interposed.
27 is "mallard" but the "tail" is changed so that a letter on the front will give a two word description of a gun.
25a: Ballesteros refers to Seve, the golfer, so your surmise is correct.
Merry Christmas