Always need to throw daggers at Dahlin....
As for the pictures
I agree, there was a pick play which effectively took Tuch off Keller
Once the rub had worked and Keller was going in on Quinn, Quinn needs to step up on him with Tuch in pursuit. Not back away towards Dahlin who is positioned to cover the backdoor. Had Keller passed it to a guy who scored then Dahlin would be more at fault but Keller just drives the net. Dahlin realizes Quinn backed off and does make a last ditch effort to get to Keller but he's too late at this point.
As with defensive play in general, when playing a 1v1 with trailing support you want to either slow or angle the opposing player away from the net allowing the back checker either time to catch up or cover the possible trailer.
In that particular case it would be Quinn's responsibility as he was the "LHD" at that time to take on Keller; Dahlin would then need to adjust accordingly based on the potential trailers coming in. Ideally in 3v3 Quinn steps up onto Keller and sandwiches him with Tuch causing a turnover or at least no shot as it slides harmlessly to Lyon. Dahlin is trapped guarding against the trailer incoming; he can't commit to Keller unless he effectively abandons his position to puck hunt.