Sharon,
You've gotten a lot of great advice on here. The 2-1 was tough for my DD as well, especially early on!
Here's what I've noticed. I do think she's clingy because she's really tired, just as calebsmummy has said. My DD acts the same and did especially at the beginning of 2-1. Also, especially after today, I think the long am isn't working for you either, like henry1 mentioned. Unless you are able to force that pm catnap to happen, like in the stroller or in the car, I think it's going to be hard to get it to work, because she keeps refusing it. So, after today's nap, I think you really would have to do a quite early dinner and bedtime at about 6 pm. It does sound scary, just like Mommy Mertel has said, and I was anxious to do it too, but the first time I did was right around 15 mo and my DD slept almost 13 hrs. It really helps get out of and prevent OT.
The other option is to obviously do the short am, long pm. I fought this for the longest time, because just like you I could never figure out when to put my DD down for the second nap as her cues were very unreliable after a CN. So, I kind of had to work it by trial and error (not really my cup of tea, but worth it in the end). I knew if I put her down early she would most likely roll around and then fall asleep close to the right time for her. But if it was too late, she would throw an OT fit. So, after a couple days of putting her down a little early, I was able to find the right time. This was, and really still is, the best way to get out of an EW or OT for my DD. She sleeps much better at night after a longer PM nap.
I think today you probably put her down significantly early. What was her latest consistent morning A? I might try around 3.5 hr A in the morning followed by a 20 min CN. Then go for about 2.5 hr A after and see what happens. She may go straight down and then you'll know, or she may play around for a bit and then fall asleep. After a couple days you should pretty much be able to guage the right time.
I really hope something in here helps. It is a hard time, I remember it well. Lots of hugs!