Totally agree about the "wanna hike" texts, they have always seemed out of character to me also. And the fact that he throws "hiking or biking with the wife" around several times to MG, LE, etc. Seems like that was part of a made up alibi. Had BM EVER biked with Suzanne, ever? Did he even own a bike?
But yes, to answer your question, he could totally have texted from another device to make it look like it was SM. All you need is someone's iCloud password to do such a thing. I think he was keeping said device somewhere down by the river, perhaps in an outbuilding or something and he would go out there to spy. Problem is that even though an iCloud password allows you to see texts and email, it doesn't let you see things like browser history or app use. Only the phone itself allows for that. So he wasn't really able to find anything that way.