This is because said external force left Elly somewhat traumatized and destroyed her common sense owing to its outrageous behaviour. Since Elly has a little bit of PTSD, she won't really back up most punishments because she fears the return of a nightmare scenario: a child screaming his fool head off for years and taking for-Goddamned-ever to admit to himself that life isn't a plot to ruin him.
That's right. The external force I'm talking about is human garbage who thinks that verbal effluent like "And the living buried the dead" is the apex of wit. Had Mike not been such a vindictive, small-minded, short-sighted and moody cretin, Elly would have had the confidence to handle a moody child. What this means is that everyone has had it wrong for twenty one years and counting. It wasn't April's naughtiness and disobedience that killed Farley. It wasn't everyone else's being asleep at the switch that did it. No. The person ultimately responsible for the disaster was miles away hanging with Josef Weeder. Simply put, Michael killed Farley because he made Elly a worse parent than she would have otherwise been.