I initially believed SM might have really planned to leave his wife or already had when he called Heather and told her as much. I believed that at some point during night, TM found out that the two of them were together & that she probably followed SM when he left to meet her (and/or tracked his phone/car, etc.) OR she intercepted phone conversations/PM's and planned to "fix the problem" for good.
However, many of you have brought up a few good points that make me think otherwise. IF Sidney was ONLY involved because he helped Tammy cover it up... (maybe he feared for his life or maybe he knew Tammy would blame it all on him... the evidence did point in his direction because he was the last person to talk to her, after all...) I think he would have talked by now.
I think we all can assume Tammy has some type of hold over Sidney. Their relationship and his kids are all he has in life. She's completely cut him off from his parents and I doubt he'd be able to afford the many trips or lifestyle he's grown accustomed to over the past 17 years. Tammy would probably do everything, and anything, possible to gain custody of their children if they got a divorce, and she would probably make it really difficult for him to even see their children. I'm sure he was well aware of that. She probably even held it over his head every day. His whole world 100% depended on this woman, and she did a great job over the years to make sure that it did. I have no doubt about that.
With that being said, I really do think Sidney cared about Heather. Yes, part of that logic is because I hate to think about him harming or using Heather intentionally. But also, I don't think Tammy would have hated Heather with so much intensity (enough intensity that she'd KILL her) unless she knew Sidney truly cared about her.
I think we can safely assume the affair began as far back as July (from HE's tweets) and possibly even as far back as March (based on HE's tweet about needing to be in FL because the distance was too much... and we know the M's were in Florida in March, as well). If the affair had already began back in March, and it ended in Sept/Oct as Tammy mentioned in her first FB post, that's 6-7 months. If it didn't end in October like TM thinks it did, you're looking at 8-9 months. That's a significant amount of time.
So now, I'm leaning more towards TM finding out that they were still in cahoots somehow. She threatened him with divorce and custody of the kids, (and possibly castration because she's a crazy lady) unless he gets rid of her for good. He complies, and they came up with the plan to lure her to PTL and murder her.
I just fear whatever they may have done to hide her body might be one that cannot be recovered. I know I keep forgetting about the amount of time that passed before anyone was aware she was missing. If the murder occurred between the hours of 3-6 am on the 18th and her car wasn't located until 7 pm on the 20th, that's like 50 some odd hours before anyone was on their radar.
I just keep replaying different scenarios in my head... trying to figure it all out. It's making me crazy!!