This is an answer to your second question. As noticed by Steve Bleiler in "A note on Unknotting number", the minimal crossing diagram for the two bridge knot 10_8 ((29,5) in two bridge notation) has unknotting number 3, while a 14 crossing diagram has unknotting number 2, which is the unknotting number of the knot.
EDIT: In looking at the literature, this observation is also attributed to Yasutaka Nakanishi, "Unknotting numbers and knot diagrams with the minimum crossings," Math. Sem. Notes Kobe Univ., 11(2):257–258, 1983.