I remember just before we went to 2 naps, we also did long morning nap followed by 2 short naps. We decided to simply accept the 40 minute second nap and move on from there. And strangely enough, the catnap became a lot easier to get. Moving to 2 naps also means you only have to stress about 2 naps :p
Nap resistance usually means UT. Give it another 10 minutes and try again. By 2.5 you means 2 1/2 hours right? So then that is the right time. Maybe if you try for PD just 5 minutes before hand instead of 15, she might go easier? I wonder if her times are actually longer than that since after a 45 minute nap, she then did 2.5 and slept off but after a 1.5 hour morning nap, you got resistance. Something to consider....