OK, so sounds like she's not *too* OT any more and she's getting a decent amount of sleep at night.
As she's done OK with an 11am nap/5.5 hrs A time in the morning today, why not stick with that tomorrow and for the next couple of days? Ollie has done better since I've gone on clock times rather than A times and fixed his nap at a particular time - I think it feels more consistent to him (which I didn't expect, given that his wakeups were all over the place!)
Initially I fixed his nap at 11am and planned to shift it 15 mins every 3-4 days. It didn't really happen like this tho, cos we were still getting loads of NW, erratic wakeup times and short naps so he didn't really settle into that routine. When I jumped his nap back to 12pm, then we started making some progress as I kept his bedtime at 5:30 and he finally started catching up his sleep as it was the pm A times that were the problem (think he was more OT than Lily is now tho).
Sooo, we got into routine that was 5:30am wake, 12-1:30pm nap, 5:30pm bed, asleep by 6.
I then shifted his nap to 12:15 and his bedtime to 5:45, expecting that it would all just shuffle on a bit. But no! What actually happened was that he took a long time to get to sleep that night, finally going off at 7:20pm, and then he slept through until 6:45! Since then, we've been on a 6:45-7am wake, 1-2:30 nap (or even 3:20 as it was yesterday!), 6:45 bed and asleep by 7:15.
In the end, we've not gradually shifted, but just made a jump back into the land of the living! I'm amazed that it happened like that but it seems it is possible

Maybe it's too early to be doing a victory march, but we've not had a wakeup before 6:15 for almost a week (before that, we'd only had one AFTER 6:15 in almost two weeks!) and his naps are consistently 1.5hrs or more.