Is it even possible that there was no mistake? Everyone seems to think BL used GP's phone all the time, so why not consider that he simply texted GP's mom because he wanted Stan to stop contacting them. If he was using her phone on the 27th (either before or after her death), which was not unusual, then to him, writing it the way he did made sense. Without knowing the time it was sent, or the time of her death, we can't possibly determine the source or reasoning behind this text. I figure BL didn't text NS from GP's phone regularly, so to NS it appeared to be coming from GP, making it "odd." JMO.
Regarding the 30th text, what was it the coroner said during an interview about this? I thought he corrected the guy interviewing him and said, "yellowstone." I saw this mentioned days ago...
Also, we don't know when the snapchat came on the 27th, do we?