It definitely sounds like your lo is ready to start increasing her A time. Yes I would do about 15 minutes every few days and see how your lo copes. Watch her cues very closely, you want to make sure she does not get overtired, as this can cause very short naps too.
I think it is quite common for lo's to be grizzly around that sort of time. It is a long day for them, even when they nap well. My DS is 18 months, and he still gets cranky at around 5:00 ish.
Good luck with extending the A time, let us know how you get on.