Oh, I feel silly now, you already posted that, sorry.
We were stuck at 11.5 until I just threw in the towel and decided I was going to stop trying to manipulate his sleep. The first thing I did was one day when he only took a 1h10m nap I put him to bed at 6:45 that night. That has never ever worked for Lyle before, and sure enough, the next day he slept in. I guess he was just so tired. And then, that day I thought for sure I would get a nap refusal that day because he slept so much at night, but he went down for his nap after a shorter A time then normal and slept 2 hours.
My gut just says he's in an OT loop again because of the SA when daddy was gone. And I say that because there are lots of days when he only gets 1 hour of day sleep, and you are still having hard nights.
Are you still doing an earlier bedtime on nursery days when he only naps an hour?
I've been trying to keep that last A time to no more than 5.5, every now and then it will be more, but not several days in a row. That seems to build up the OT.