Yawns are not necessarily reliable tired signs - they can mean a baby is bored, needs a change of scene, or just feels like yawning! Some babies really love to yawn! For her age you're looking at an A time of about 1 hr 20-30 minutes. There's nothing wrong with a bit of clock watching if it helps you figure her out.
Stopping feeding to sleep is trickier proposition, but she's very little and can easily learn new things. It depends what you want to do. You could start giving her the 'snack' a while before her nap time, so you know she's fed before her nap, or you could try to cut that second feed completely. It's up to you, but it sounds like you might be uncomfortable just dropping that feed, and that's perfectly fine. I know mine needed to have a 'snack' when they moved to 4 hour EASY. What you could start to do is make sure she isn't feeding to sleep - so feed her, and rouse her up before you put her into the cot. She doesn't need to be wide awake, just not sound asleep after feeding going into the cot. That way, she'll get used to going asleep in the cot without feeding, iyswim.
You could also have a look at the bfing board, if you want to chat more about bfing. Are you offering both sides at that first feed? That would be a good way of differentiating between 'meal' time and 'snack' time.
HTH and good luck