IMO, the formula for every theory is pretty simple: Behavior + Timeline + Evidence (scientific).
Behavior.
KC's behavior didn't change. Nothing different between 'old' KC and the 'new' KC. A) KC was always the LIAR. She lied regarding her pregnancy, her work, her 'college'...and of course the 'nanny'. And she lied to everyone: her friends, her parents, LE...the reason why her friends couldn't see these lies because they're not care much about her work/college/'nanny' stuff...they meet to have a good time, to party, to drink...plus they're young. Why bother to pay attention to such a 'boring' details? However, her parents knew about (remember what Cindy told Amy in the car?); B) KC was always the SEX/PARTY girl. She's only 22 but look how many male partners she had, in how many 'serious' relationship she been involved. And how many one-night-stand she has (we only know about the fired police officer and possibly the real Caylee's father)? Her pattern to line-up the next boyfriend at the same time she has the current one is the 'stable' pattern for long-time; C) KC was always the THIEF. IMO, she was stealing money from her parents for long time. She even used her 'close' friends to 'milk-out' the money. She use JG to get $400 for 'rent', remember? You'll say it's not the theft. Yes, it is! The use of false pretend to gain the financial gain is the theft! And when her parents caught her with the credit card and grandparents thefts - they probably cut her off completely. So, she continue stealing money the 'hard'-way, without the scariness to be caught. Amy's checkbook was the convenience and she used it, no guilt, no regrets.
D) KC was always the clever MANIPULATOR. I'm not going to waste the time on this one. Just listening to her phone calls from jail, her responses to LE, reading her facebook comments...I have no doubts that she is and always was the clever manipulator.
Timeline.
We do not have the complete timeline from June 15 until July 15. We know the certain events but not the accurate TIME and LOCATION of ALL EVENTS. The 'Timeline' tread on WS is an excellent start. But a lot of holes there because none of us have the full documents of the phone records, credit cards receives, banks transactions, surveillance tapes and so on. I'm sure that LE/FBI collected them as much as possible or at least working on this topic to make sure that every hour and location of Casey's 'journey' from June 15 until July 15 is accounted for.
Evidence (scientific).
This is the most important factor of the theory's formula: 'premeditate murder/accidental death/manslaughter' versa 'kidnapping/sold/hide' theories. If evidences are strong then 'premeditate murder/accidental death/manslaughter' theories are the one to explore. If not then 'kidnapping/sold/hide' theories are applicapable. And we can speculate either way based on the 'leaks' and/or LE/FBI 'possible' statements. IMO, if LE/FBI doesn't have enough evidences in support of 'murder' - KC will be charged with 'child neglect' and other fraud charges in November to keep her in jail as long as possible. The 'murder' charge has no time limitation therefore LE/FBI could do it in year or two, when they're ready.
Now, because this tread is about theories, here is my theory (based on what I know today, of course). June 15 is the 'turning point' in KC life. No more financial supports from parents, no 'home', no place to stay for HER. But not for Caylee. Her mother wants/cares about Caylee and Caylee only. Her mother would support and provide home for Caylee, she will adopt Caylee. The ultimatum was made on June 15. KC is on her own from now on!. She runs from home crying, calls TonE and stays at his apartment over night. (I always wondering, why wouldn't she went to Lee? Interesting to see if she called Lee on late evening of June 15?). Caylee stays with Cindy. On June 16 morning, KC came back home after Cindy went to work. She sweet talked to Daddy, told him what kind of plans she has (something like, I found such a nice apartment for me, Caylee and TonE...or I'm getting engaged with TonE). Takes Caylee and leaves the house (exactly as GA stated). Few hours later, she called GA and CA on cell, at work and at home to make sure that they at work and nobody home. She returns with Caylee home. Her plan is to PAY BACK (punish!) her mother. You don't want me then you'll not going to have Caylee!!! If you only cares about Caylee then you'll pay for it! IMO, KC killed Caylee (how? I don't know, many possibilities exist). For KC it was important to do it at Cindy's home! KC action was based on her anger, emotion and egoism. However, after she completed her plan and saw the dead Caylee - the reality hits her and she panic! The rest of it is just the cover-up by the person we know: Casey the LIAR, Casey the MANIPULATOR...and all of the above. Sorry for the long post.