Ooh, she's a tricky one, isn't she?!
I think I'd be tempted to try one or both of two extremes, just to get a clear reaction. If she's getting OT then I guess you could go all out on fixing that, try a day or two of a completely uncapped nap at 12 and an early bedtime of asleep by 6.45 or 7. I know you're worried about what that'll bring, but if she's OT then the long days are going to be contributing to that, no matter what you do with her nap. I'd get an early night myself, prepare for the worst and cross my fingers....!
The other extreme would be to cap at 45 minutes for a couple of days and see where that takes you. If she'll nap in the car or buggy you could even have a day out as a break from all this and try and distract yourself! I'd probably bring BT forward in that scenario, too - maybe aim for 7ish there as well.
If neither of those work I don't know what to suggest I'm afraid! I think you've tried everything else, so I'd probably push BT right back in the hope for a more reasonable WU, set a nap and BT and just observe her for a bit to see if anything emerges. Sorry, I know none of those will feel like options but like I say, I think you've tried everything else...