a. Competition from an NHL regular is a little different than competition from a guy like Adam Clendening. Moore's 5 year contract was a bit of a head scratcher, but the cap hit is low. Regardless, he was one of the better value pick-ups this free agency. Also, the $3-$5 in the press box is nothing to worry about considering the Bruins will be very efficient with cap usage elsewhere in the line-up. Also, the $21m devoted to D is much lower than most teams.
b. This is the hardest point to refute because you are right, Moore is not statistically better than our other LHD. However, Cassidy's focus on transition game and skating lead me to believe that Moore has a good chance of finding a fit here in Boston. He is certainly not a defensive defenseman, but he had a strong 2018-19 and can log 18-20. Remember when the Bruins picked up Nick Holden..and all the hate he received. He was fine. Pair Moore with a defensive stalwart like Miller and see what happens.
C. See point A as well. $21m is not an overpayment on your D squad, especially when you only spend <$6m on your top pairing. Bruins can afford to invest in quality depth in 2018, especially when they were the 2nd most injured team in the NHL last year.
d. Until we see how the left side looks this year, how can one decide whether Krug, Gryz or Moore should be traded? Again, with only $21m tied up in the D corps, I don't see why you can't keep them all on the roster this year.