Okay for 1.) she's best friends with your ex girlfriend. This in itself Is a really hard to over-come because, if she values the friendship with that girl at all, she's not going to date you, nor hookup with you. You should also be careful. Birds of a feather flock together, & according to assortment theory, you're friends with people who are like you, share the same ideas, values, etc. So if her and your ex girlfriend are best friends, they're more than likely very similar with their ideals. A.k.a. She's like your ex girlfriend & why would you want a girl like that. It's not a deal breaker, girls think emotionally so you could pull it off if you build massive value & attraction in her eyes & have her backstab her friend, but it'd still be hard.
2.) You don't see her in person, so every interaction is over text. This makes it SUPER HARD to do anything. Texting is a 3rd tier form of communication, & it doesn't even compare to real life interaction. You can still build attraction over text, look up T-MAL's guides on texting. In my opinion, texting to build attraction/rapport sucks & you should only use it to set up logistics/ask to hangout, or to share something playful & high value. By the sounds of it, it seems like you've been made into her little texting buddy becuse you didn't build any attraction.
3.) She straight told you that it wasn't going anywhere because she values the other friendship with the ex girlfriend more. There's you answer. She's really flirty because she loves the attention you give her, & she doesn't feel any attraction so it boosts her ego and makes her feel good, aka, being a flirty emotional play girl like AXle said^
4.) she flakes because she feels no attraction, & I'd counter-argue with this dumb bitch of a girl, that if she just wants to be friends, that's cool, because that's all you want too. But friends hangout & spend time together, & if she doesn't want to be my friend then later because this isn't friendship. THEN see what she says to you sticking your foot down on the situation.
5.) don't tell a girl to "get more available" In hopes that she'll magically make time to see you. She won't, ever.
6.) their is no such thing as the friend zone. I wrote an entire thread on it in the ask an expert column. She's just not attracted.
Basically, she's not attracted to you. In my opinion, it's because you came on too strong, were to available/low value.
+ the ex girlfriend situation hurt you chances.
I don't think there's anything you can do except try to stick your foot down on the hanging out, or to try & somehow build attraction over text & show HIGH value. Very tough, but you could do it.