I can see why she would have a cat nap at 5.30pm, because of the time she wakes up from her nap. I think if you wan the 6pm bedtime, you need to stay with the grumpy child, as the only way to change it would be to get her up earlier (almost 13hours overnight is very long) & move your day forward to fit a catnap in.
That said in a month or 2 she will probably be old enough to cope with the longer A times without getting grumpy.
At that age my DS2 had a routine of a short morning nap, a 2.5-3hour mid day nap & a short afternooon nap as it fitted around his brother, it was something like wake up 6 am, 8.30 45min , noon 2.5hour nap, 5pm 45min nap & 7.30pm Bedtime. As you can see his night sleep was only around 10.5hours, where as yours is almost 13 hours, so it's a matter of juggling the times.