Based on the MPR, isn't it "fact" that Diena texted first? At about 4 she texted Sean and asked how the kids were doing and he texted back that Somer hadn't come home with the other two?
If asked when she first realized Somer did not come home, she would probably say that she got a text message from Sean. I don't think her omission that she texted him to ask about the kids is a red flag (necessarily).
When they questioned Sean and he described his actions, it would have included telling AT to search really good before he called Diena. At about 4, Diena texted him and he responded that Somer hadn't come home. I think he also said that AT was looking for her. It is possible that Diena didn't want to put Sean in the position of being the adult in charge and didn't want AT to be the one out looking for Somer, and that's why she left right away.
By the time she got to OPAA, and Somer wasn't found (cell phone records should show calls (text or phone) between her and Sean, she felt something was wrong and flagged down the officer. Does that make sense?