I don't think you are misinterpreting the texts. Though such a plan is possible, as you state, it also flies in the face of any experienced SOCAL resident and even any sense of travel efficiency:
She would need to have planned to....
Drive from Ontario, back down to San Diego, only to return to Los Angeles County the next day. Then take a winding, crowded road into the mountains to Big Bear, spend Sunday night. Then, get up, take another less busy, but still winding mountain road east down to Palm Springs. Then leave the same day to fight very heavy Labor Day Monday traffic all the way back to San Diego.... .
In short, I think you are right. Whoever sent those texts was referring to driving from Ontario to Big Bear Saturday night, then spending the night (which, if done impulsively on a holiday, also flies in the face of any SOCAL resident)