To me this sounds like a manic episode. I've been through one with my brother and it's terrifying. You also have to remember that all/much of what the manic person is doing seems irrational.
I'm guessing she took off for Myrtle Beach without telling anyone. Maybe she did plan to see her dad, maybe she has friends in the area, maybe she just felt like going somewhere. But she's in college and this didn't happen during a break so there would be no reason for her to be driving for 8-9 hours. She gets there and for whatever reason goes to a scooter rental store and gets confrontational when she is denied service for not having shoes. Seems the cops may have been called. She calls her mom (or the cops instruct her to call her mom). Her mom finds out she is in SC, recognises she is manic and tells her to go to her father's. It is not safe for someone in this mental state to drive alone at all, let along 8-9 hours.
Given the timing of everything I'm guessing she takes off from the scooter store, maybe to drive back to FL or maybe to go somewhere else entirely but not to her dad's. Manic people tend not to take kindly to instructions. Now, do we know the car was actually "torched" or is it possible she crashed it and it caught fire. I'm thinking she's speeding away from MB, loses control of the car and crashes it into the cornfield. Possibly the car catches fire. Possibly she panics and torches it herself (there is a good chance she was also under the influence of drugs).
Now how does body get to where it was found two days later? She could have walked. If she was manic, she would have a ton of energy to get where she wanted to go. Maybe she walked there, was tired/dehydrated/took something and passed away. But I'm inclined to think she ended up meeting someone when she fled from her car and possibly she overdosed while with them and they dumped the body. Or something else nefarious happened.
All of this is just speculation, of course. There's a lot we don't know, especially about the time after she left the scooter shop. RIP Sheridan.