I'll play along with your theory it's the boyfriend.
I'm a bit perplexed by her returning home from a date and ending up in the bedroom without having a sexual encounter. That makes me wonder if she was dating more than one man. Why not? She's separated. There's no commitment to this married boyfriend.
We know nothing about Sarah. She could have been a great actress. There are indications she was an addict and may have been in an abusive marriage. Prior to reading the articles, I already suggested the possibility that she made a bigger deal about her pre-cancer for attention. It's equally possible she's a passive/submissive personality. Maybe she had a date with a new guy and didn't know how to tell the boyfriend to go home. She plays sick and he leaves.
Let's consider abused women tend to be attracted to abusers. She finally got rid of her husband, and now she wants to distance herself from the married boyfriend. After all, he's married. She wants to see other people. This married boyfriend was good for helping her get the courage to leave her husband, but he's not the future she envisions. Maybe she's noticing possessiveness, maybe not.
So she fakes being sick, and goes out on a date with a new guy. They return to her house, but they don't have sex because it's a first or second date. When he leaves, she goes upstairs and is ambushed by the married boyfriend. He shoots her because he's furious she is seeing other people behind his back. He truly did envision a life with her and her money, but now she's better off dead to him if she's going to see other guys and leave him to live the hard life of a trucker by himself or stuck married to his wife he can't stand.
Now that gets my attention. The will alone wasn't a good enough reason for me. At this point in time, I believe he looked forward to a life with her. She's an attractive, wealthy woman. I'll get his wife wasn't either of those things. He probably did want to get rid of his wife. I doubt that was practice. It was probably a failed attempt to murder her. After the restraining order, he couldn't pursue that anymore. And if he ended up with Long, he may have eventually done the same with her. You know those men and women who leave a few spouses dead under suspicious circumstances and nobody realizes it until a few of them die. But on this night, he didn't give a sleep aid. He was waiting in her bedroom and shot her.