So there is a thing called ‘spoofing’ which essentially lets you impersonate someone else for text, calls or chats.
Spoofing Evidence — Technology Safety
This my MOO, but I don’t believe the Saturday morning texts between BM and SM were between them. I believe they were between BM and BM (impersonating SM).
It would be too much of a coicindenxe for SM to request hot tub stuff (ie Chrlorine). I think BM created the hiking scenario to make it seem like all was well, and the hot tub store scenario to give an alibi for why he needed chlorine.
Yet, it was nothing more than his premeditated plan to kill SM that afternoon. And this why he was acting so off to MG Saturday morning.
Edit to add one more link on this topic of spoofing and criminal cases:
Text messages require expert witnesses (Four reasons) - Solomon Criminal Defense
It is common in domestic violence and I’m sure something that the FBI is familiar with and has looked in to. Although, I’d imagine that info won’t come out in the prelim but maybe it will.