Hiya - sorry for not getting back to you yesterday.
Yes do what you said & do 3 days at 1h 15 to start. The 2nd A time will need to be shortened as you start capping the AM nap, as she will be unlikely to manage the same amount of A time after a shorter nap. My LO could do A time of around 3h 15-30 after a 1hr 15 nap, 3-3h 15 after a 1hr nap and 2h 45-3h after a 45min nap, if that gives you a rough guide. Not to say your LO will be exactly the same as they are all different of course!!!! Its just a case of trial & error. Eventually you should find that the PM nap starts to lengthen out.
The idea is to keep cutting back the AM nap until she consistently gives you a good PM nap, AND she is not waking early.
We are doing the very same thing, and our AM nap is now 30mins (9.30-10) & he then does a 2.5hr A time followed by a 2hr PM nap from 12.30-2.30. BT is 7pm (provided he slept the full 2hrs). We are still getting EW though, so I suspect my Lo will need his AM nap cutting back to 20mins soon.
As for nursery - I would ask them if they would stick to your routine as best they can! You are their customer, so they hopefully should be willing to meet your request.
Good luck.x