My DD was just like this. She went to one nap around 17.5/18mo, but it kept getting shorter and shorter til she was an OT mess and I was forced to go back to 2 as it was the only way to catch her up. She's never been a power through it kind of girl. I got so frustrated with people telling me she should just be on one nap (she was too old for two naps), because it honestly didn't work. She would still take 2 naps even up to 2yr old, when she was teething and got OT.
I wonder if the reason she won't take a pm nap is not just the AP thing, but because her am nap is too long? DD definitely reached a point where she wouldn't take a second nap willingly if she had any more than 20-25 mins in the am, so it may be if you cut her am nap short for her she will be more willing to take a good second nap.
I think this is right on, though. If you want two naps to really work, I think you need to cap the morning one and do a longer afternoon one. Then, you can cut the morning one down to 20min and it should be easier to get to one nap. It does sound on the whole that she's never really getting a full nap, which might be leading to some UT/OT issues. The timings and nap lengths are UT, but she acts OT because it's never really enough sleep to restore her. My DD was like that too, I always thought she was OT because she never seemed to get enough sleep, but she was UT too some of the time and not doing full naps. When I finally stumbled onto a 20min am, 1.5-2hr pm nap it was like I hit gold with her.