What was the point?

I agree. McKenna's actions were deliberate and started right after the husband told her she wasn't going to babysit for the little girls. She leans back and opens her legs to him. She was hurt and was going to get back at them. She was a very damaged and destructive person.

She didn't have plans. She never had plans. She was free when she was originally asked to sit. It's alluded to during the kitchen scene that she's perhaps expected to be gone by the time the guys show up, but it was pretty apparent that she didn't actually have any other plans.

McKenna's actions might have been "calculated", but this is kinda pretty much the immature and brokenly justified reaction I would expect from a 22yo who's several times now been jilted by the woman who came to her, offered her friendship/help/assistance/etc... only to repeatedly deny/reject her over and over again at these personal and vulnerable junctions. She just kinda regressed into this old, familiar self-destructive behavior which was more primal than logical.

Also...

Rachel didn't lose anything. Those "two friends" weren't really friends, just moms from the school she hung out with more just because of their kids than anything else. She even stated this in the last therapy session. She only really had the one friend, Stephanie. That close friendship was established at the very beginning of the movie, supported throughout and at the very end she was still there for her. The other moms were actually part of the original problem. She hated doing and trying to be one of "those" kind of moms. If anything it's actually hinted at that she might have started to gain another real friend in Kosher Amanda. They had their brief, but honest little moment at the end.

She didn't have plans. She never had plans. She was free when she was originally asked to sit. It's alluded to during the kitchen scene that she's perhaps expected to be gone by the time the guys show up, but it was pretty apparent that she didn't actually have any other plans.

McKenna's actions might have been "calculated", but this is kinda pretty much the immature and brokenly justified reaction I would expect from a 22yo who's several times now been jilted by the woman who came to her, offered her friendship/help/assistance/etc... only to repeatedly deny/reject her over and over again at these personal and vulnerable junctions. She just kinda regressed into this old, familiar self-destructive behavior which was more primal than logical.

Also...

Rachel didn't lose anything. Those "two friends" weren't really friends, just moms from the school she hung out with more just because of their kids than anything else. She even stated this in the last therapy session. She only really had the one friend, Stephanie. That close friendship was established at the very beginning of the movie, supported throughout and at the very end she was still there for her. The other moms were actually part of the original problem. She hated doing and trying to be one of "those" kind of moms. If anything it's actually hinted at that she might have started to gain another real friend in Kosher Amanda. They had their brief, but honest little moment at the end.