First off, well done on doing the work in getting clips for the post.
As for "On the other hand, you could play good defence against Bossy and he could still beat you with his incredible shot and release" you're going to have to show me the ones where the defence is playing well.
For Bossy, I'll put forward the following:
Goal 2. The Islanders are moving the puck well on the power play and the Bruins are chasing but I don't see anyone out of position. Bossy finds a little space in the high slot, so the one high forward could have covered him more closely, but it's a well timed pass and Bossy gets the one timer away with what looks like not much space and no time at all.
The last two goals in game 6 aren't terrible D. Bossy just beats the defender to a rebound both times, and does a lot of work for it the second time. The defenders are engaged even if they lose the battle.
For the rest:
Goal 1: Krushelnyski chases Jonsson too far and leaves Bossy wide open. Or maybe Brad Park should have rotated up? For that matter, Ray Bourque had just been on the boards with Bossy before he skated back to the front of the net to help Park cover Trottier and Kallur. Not sure who their coach would point to in the film session.
Goal 3: Hillier gets caught playing the puck, gets turned around, and loses Bossy during the one-two.
Goal 4: Multiple Bruins get caught in between, looks like they didn't expect Jonsson to make that pass.
Goals 5 and 7: Bourque gets caught flat footed and Bossy gets a step on him both times
Goal 6: Peeters gives it away
Gretzky's goals had some pretty bad mistakes though. None as bad as Peeters' giveaway, but several where a defender just takes himself out of a play and I can't understand what they were thinking. To me they look worse than the Boston errors because they're constantly leaving the net unprotected.
Goal 1: Why does Doug Crossman just skate away from the net with Coffey after Coffey centres it? Why does Poulin just turn away from the net instead of staying with Gretzky, leaving Gretzky to beat Howe in a race for the rebound by the open net?
Goal 2: Coffey and Kurri run this quick, tight cycle in the right circle. Well done, good play by them. But Thomas Eriksson follows Kurri to the top of the circle and then just disappears. Where did he go? And why is Dave Poulin just standing on his heels in the high slot without engaging with first Kurri and then Gretzky skating uncovered to the net? I don't think Huddy is even at the left point behind him.
Goal 3: Poulin catches an edge while skating with Coffey, and takes himself out of the play by skating behind the net. Which forces Howe to skate out after Coffey, meaning Crossman has to cover the net alone, but Crossman doesn't realize Gretzky is behind him, stays too high, and lets the pass get between him and Lindbergh.
Goal 4: Good play by Messier, overlapping with Coffey and forcing defensive decisions. The Phillys RD steps up to take Messier, the F1 backchecker tracks back on Messier, and nobody takes Gretzky as he goes wide. Main mistake is by the Philly RD (or forward? Possibly 3 forwards at 4-on-4) who stepped up on Messier and let Gretzky get behind him.
Goal 6: Crossman and Ron Sutter both commit to covering Glenn Anderson as he drives the net. Bad mistake when killing a 5-on-4 as they both go behind the net, and the worst happens as they let Anderson hook it back into the open space they vacated.
For the other two, first Crossman and then Howe both just gave Gretzky a bit too much space around the net on each goal.
I will try to watch a Boston-Islanders game this weekend, but it seems from the clips that the narrative from the UPI article about the Bruins being lost on big ice makes sense. Peeters had a great season with a 1.82 GAA, but he's posting a 7.67 GAA on the Island in this series.
I am not qualified at all to evaluate 80s goaltending but I could totally believe Peeters had a bad series, based on these goals.