I've lightly flirted with her in the past, she occasionally flirts back.
I can assure just from reading that text thread that she has not been flirting back with in any meaningful. She's beta/orbiter-zoned you, and she sent that snapchat looking for validation and compliments. It doesn't matter if she's sexually inexperienced, AF/BB strategy is instinctual for women. Unless you raise your SMV substantially in her eyes, the only future you have with this chick is as her orbiter, so it's best that you cut cord, radio silence and move on.
Some thoughts on your game: It's been often said on TRP that the best game is having an awesome life/high SMV. This girl is clearly not attracted to you, so your SMV could use some work. Take 6-12 months, go into monk mode, lift, focus on career, take up a new hobby, and work on day game approaching.
On your texting... I won't lie to you, your text game on this thread has an extremely lurching, creepy feel. I think you correct this by changing a few simple things.
Her: slutty pic
You: (two hours later) You need to clean your room/your mirror is dirty.
This will set you apart from the six other betas that she sent that pic to who are hungrily telling her how how she looks.
Don't respond so quickly man. This is PUA 101. On that thread, all your responses are rapid fire, 1-3 minutes after she texts you, you send back. Cool it on the texting. Find something else to do besides talking to her and make her wonder why you aren't responding/what you're doing/are you texting some other girl?
Avoid using childish words like "tummy." This type of language feels warm and fuzzy, and will just cement her image of you as a beta.