I couldn't decide if I actually wanted to reply, but here you go.
1) Demeanor - You are absolutely right that we cannot comment on the demeanor of an unsub. However, we can comment of the demeanor of any person listed in the SW, and from what I have seen, the in-laws collective demeanor is not consistent with a grieving family. First, you have RB gloating over and laughing about dog blood four days after his son's wife was brutally murdered. Then, you have KS talking about MB's affairs, and MT is no better. There have been numerous comments about how this affects MT, KS, RB, BB and so forth, but there is very little talk of MB and what her life meant. You also have numerous examples of peculiar comments that don't really seem to add up. When taken as a whole, the behavior is suspicious.
2) Motive - You have a husband and wife suffering from "martial and financial" problems. That is motive enough IMO. There are several other possible motives as well, but those cannot be mentioned here due to TOS. You can sleuth those on your own. On top of that, we don't know that life insurance wasn't a factor. We don't know that a divorce wasn't in the works. As for the potential current affair that you reference, we don't know all of the details. We know that MB was currently exchanging familiar and flirtatious emails with at least one member of the opposite sex. There are also other people listed in the SWs who are likely there for questionable relationships with both MB and BB. However, we don't know the exact nature of those relationships so speculation is the best we can do.
3) Means - You are absolutely right about other people having the means so kill MB, but that doesn't change the fact that the people close to Missy also had the means! Also, I believe that this was a plot that involved two or more people. Since so many people close to MB (not just BB) had motive to kill her, it's not a stretch to think that they would plot together to make it happen. I am not sure why you jumped to the conclusion that this means that Missy "could have only [emphasis added by me] been killed by someone she knew." In fact, I don't think that at all. However, with the limited information that we have (combined with the information we are allowed to share on WS), that is my top theory at the moment. Of course, if more compelling data is brought to my attention, I will happily consider it. Having said that, my list of potential plotters is not limited to the people listed in the SWs; it's just that the TOS prevent me from mentioning the others.
With the facts that I have at my disposal, a targeted hit by someone close to MB is far more likely and logical than a perp dressed in tactical gear who killed because he/she wanted a sandwich at a church in the middle of the night. JMO
If you are asking why SP didn't go to the SW corner immediately, it's because he knew he had time. SP was prepared and knew MB would not be arriving for several minutes. SP only went to the SW corner close to when MB arrived. That way he could ambush her. According to LE, that's exactly what happened. I don't have the exact quote, but it was something to the effect of MB was seen walking toward the location of SP.
Additionally, if I look at what LE has said and done, nothing that they have said contradicts any of my thoughts. I have previously outlined my thoughts on what LE said in the May 20 presser. I will add that when LE said that Missy's family is not at the focus of the investigation, I think that they meant exactly that - Missy's family. You don't have to agree with my interpretation, and I admit that I could be wrong. However, until I am presented with a more compelling theory, this is where I stand.
Having said all of that, I can't help but think of Gary Condit (the Chandra Levy case) as I investigate the MB case. Gary Condit had a suspicious demeanor, a strong motive, and definitive means to kill CL, but he was eventually exonerated. I don't think that it is forgone conclusion that the family is involved, but with the information that I have right now, it's the most logical theory for the time being IMO.
ETA - I am not answering any follow up questions to this post. I am moving on.