Boston just should have dropped down a few spots, got a few more picks, and then took their guys. Perception would have been better I think but obviously there was either and or no deals to be made or didn't want to risk the chance of their guys being gone.
Even with assuming that those rumors were 100% true, Detroit and Toronto had picks 19 and 24 at the time. Would have made much more sense to trade back 2-5 or maybe even more spots and pick up assets while still getting the player they want. Also would have made sense to pick a guy like Barzal or Connor at one of those picks and trade up with their 2nd round picks or other assets to try and take one of their targets.
For all we know, Boston did try to trade down but maybe the fish were refusing the bait? The Kings got flack for taking Thomas Hickey in 2007 and most fans thought they could move down. Later, Dean Lombardi admitted that he tried like hell to move down but nothing materialized.
We saw from the Columbus behind the scenes videos where they were offered a couple deals to move up, but they decided to keep their picks. Same sort of thing could have happened to Boston.
Looking at the draft board:
16. Edmonton - Probably not that interested in moving up one slot.
17. Winnipeg - Had picks 25, 47, 78, 108. Plausible that Boston might have contacted them, but maybe they had 3+ guys left on their board that they liked and felt comfortable with staying put.
18. Ottawa - Had picks 21, 42, 48, 109. Maybe Ottawa would have been skittish about trading a 2nd to move up?
19. Detroit - Red Wings had 73, 110. Maybe not enough ammo to move up.
20. Minnesota - Wild had 50, 111. Maybe not enough to tempt Boston to move down.
Naturally teams could have offered up 2016 picks as well. But the Jets and Red Wings already moved their 2016 3rd. Minnesota moved their 2nd and 3rd.
As an armchair GM, I also would have tried to move down and get Senyshyn but I also recognize that the offers could have been underwhelming.
Trading back into the first round might have been possible. Toronto moved down twice and essentially moved #24 for #34, #61, and #68. Not sure if they would have moved the pick to Boston for a comparable package (Boston had #37, #45, #52, and #75 to work with).