Sorry, I've been away for ages. Things have been really hectic here. Sorry to hear you're still struggling though
So just like always after about 3 days of 1.5hr long naps and 10hrs20m long nights she gets more and more OT and then takes ages to fall asleep and is up early.
I wondered, what makes you think she's getting OT? I would have thought that after 3 days of good nights and long naps that she'd have caught up on any OT, and any further short nights would be UT. Certainly Jack would be. I know you're capping her nap slightly, but if 15/20 minutes more sleep means she takes an hour and a half off her night then she must be right at the maximum of how much day sleep she can have, so I'd be very surprised if she was getting OT from missing that amount of sleep each day.
When Jack has done this in the past, he's needed a few days to catch up (so three, for your LO), and then when he starts shortening his nights it means he needs a further tweak to maintain a proper amount of tiredness. I tend put him down for his nap at his normal time if he EW's (so he has a longer first A), and then cap slightly further. I know that sounds scary, but it's about getting enough tiredness into a well rested toddler to carry it on into the night, without allowing a shortened A, or longer nap to be the way they catch up. It looks to me like you're in a classic UT/OT loop, where she has short nights, gets OT, catches up with longer naps for 3 days, then shortens her nights due to UT, then gets OT due to the short nights....
I'd take from that that a 1.5 hour nap is good for catching up on OT, but too long for every day. I wouldn't draw any more conclusions than that. So then you need to cap at less than that to see what works, bearing in mind that it might be she needs her nap capped by quite a bit. For example, Jack needs to nap for 1hr 20 or more if he's OT, but once he's rested I need to cap at 30 minutes to maintain good nights. If I let him sleep 1.5 hours for 3 days, and then still let him sleep even an hour on the days after that, we'd be getting short nights due to UT, not OT.
What do you think about it all possibly being due to UT and not OT?