Wow, this was an effort in humility - I understand logic well, but not programming. Here's what ultimately worked:

PL1: Input: Maintenance 1 (Inverted)
Action: Delayed On
Time: 210s (tested with multiple small delays first)

Socket 3 (Skimmer) - Assigned to PL1
Socket 4 (ATO) - Assigned to PL1

Interestingly, when I created a PL2 with a delay of 360 seconds for the ATO on Socket 4 as I'd done for the skimmer on Socket 3, Socket 4 would always be off. I tried both Inverted and normal, yet the socket wouldn't come on. When I assigned it to PL1, all was fine. Is it possible a socket is faulty on a brand new unit, or would the fact that the ATO system doesn't draw any current unless called upon cause a strange behavior with the logic? Interested to hear thoughts on this.

Thanks for all the help thus far. I'm still in the steep part of the learning curve.