If the linesmen can't tell, that's sufficient for me to conclude it doesn't affect the hockey play.
Cherrypicking is what the rule is intended to prevent, so as long as they can't do that then it's fine.
First of all, the intention of the rule doesn't particular matter. What matters is the effect. If it's illegal to cross the blueline before the puck, then it's illegal to do so regardless of the intention, or subjective interpretation of the play. If a defender successfully dislodges a puck from possession of another player and that player skates over the blueline, recovers the puck while their offside, then why reward the forward for achieving an illegal play and the defender for a legal one?
Linesmen didn't notice Duchene skating offside. You're saying that's a perfectly fine hockey play?
Refs don't notice the puck off the netting sometimes , miss penalties, etc. People are fallible. You're essentially arguing nothing should be reviewed. If the refs and linesmen don't see the puck go off the netting, the goalie being interfered with, the play being offside or the net off its moorings, then it's sufficient to be part of some nebulous "hockey play." This is a professional league that people do for a living (and I'm not just talking about the players, but even the arena workers no one hears about, the broadcasters, the coaches, the medical personel, etc.). It shouldn't go by a "if no one notices" rule. I'd agree with you if the call was ambiguous, but it wasn't -- it was obvious.
Either way, good game. Glad we got the win.