JMO
For the theory that the pressure of the upcoming wedding and other life pressures somehow got too much for her and she either left with a friend or left by herself, I started a list of things that may point to this.
This is just one theory and there are many other theories that may have more weight. Even LE has said it would not be like her to leave without telling anyone. So I dont necessarily even think this theory is at the top of my list but I am keeping this option open until its confirmed that she was taken against her will.
*very early on police in Utah were asking Utah residents to be on the lookout for her where she has family connections according to this link.
Police in southern Utah ask residents to be on lookout for missing Iowa woman
*Now a confirmed joke to the BF that he should propose to Mollie during the upcoming wedding. Maybe this somehow got back to her.
"Blake jokingly told his brother that he should propose to Tibbetts, his high school sweetheart, during the trip."
Investigators still baffled by disappearance of Iowa student
*certain things in her SM may be interpreted as signs that her goals in life may have been changing. This is very much just an opinion and others may interpret things differently.
*she sure was a busy person with school, a job, dog sitting, no car of her own, etc. Life pressures may have taken their toll.
*3 hour discussion with Dad on the Sunday before she went missing. This may have been normal for her to have long discussions with her Dad. Afterall, he was living out of state so maybe this was normal.
*some of the wording in some of the articles used words that could be construed as talking to her like she had an option to come home. Again this is very much an opinion only and others may not have thought of the articles in that way.