1. It is completely fine to have crushes on people you are dating, even when you are dating someone. It's about how you handle the crush that makes it okay or not. And well since you aren't dating someone right now, it is completely fine to like her.
2. It sounds like it is what it is. She likes you, but just got out of a relationship and isn't ready to date. Coming from a girl who had her heart broken recently, I can kinda understand why she isn't ready yet. Sometimes it can hurt a lot when you get your heart crushed, whether break up, rejection, ect. No matter how much a girl can like a guy, it can be hard to start a new relationship when she is still in her healing faze.
I would just take it slow, continue being friends with her. If another girl catches your eye, don't be afraid to go after her because who knows how long you will have to wait :/ it depends really how her healing process goes.
Just keep on being friendly. I wouldn't try making out with her unless she initiates it since you don't want to pressure her. Basically do what you are doing now
I know you feel like you are doing the wrong thing, but you aren't. You are doing perfect! Believe me, you being around her probably is making her heal faster than her being bitterly by herself in her room all day. Being her friend and helping her through this gives her the support she needs to heal, and plus it gives you brownie points for when she is ready to have another relationship.
And if you personally can't handle just being her friend because it hurts you, don't be afraid to put some distance. Relationships can be hard and it is okay to look at other girls too, since she doesn't own you nor do you own her right now.
Just take things slow
You ARE allowed to like her. Completely. Don't think otherwise. You'll do fine. It sounds like you care about this girl a lot and that might be the perfect thing for her and the relationship you are looking for. I hope everything works out!