Maybe she's overtired hun... could you AP the catnap or maybe she needs a little less A before the 2nd nap or a little more? She's definitely showing signs of the 2-1 switch... but until she's ready for 1 nap, she'll need the catnap to get her through till bedtime. I would be temped to put her down even earlier than 6pm if she's missing the afternoon nap. I am mostly doing the other option (shorter am/longer pm) but on days we have done 1 nap or just long morning nap and no afternoon nap, bedtime is as early as 5pm.
Will she sleep in the pram or could you drive her around to ensure she is taking the 2nd nap?
Do these night wakings correlate with no pm nap days? If you have ruled other things out (like teething or developmental), it may be from overtiredness?