It's hard to say. Your comments are certainly logical, but not everyone is logical. If the person who is posting on his FB page is also the person responsible for their disappearance, I agree that it's not wise. But, sometimes people do things like that. Personally, I think it's pretty deranged and disgusting for the person who is posting on his FB to do so, whether they are involved in their disappearances or not.
I tend to think they disappeared at the hands of someone who knows/knew them, rather than a random abductor/killer being able to take both of them. A random stranger is possible, but, I don't think a random stranger would have much motivation to taunt his family this way. This seems more personal; someone with a grudge against the couple. Someone who lost out but feels they are the ultimate winner now. "If I can't have you, no one can" type mentality.
Now, the person who is posting may just be a sicko and enjoying this revenge fantasy, and they may not have had anything to do with the disappearance at all. But, I have to ask why do such a thing? I really don't believe a stranger would get the personal satisfaction that a 'scorned' party would. I also can't see a stranger bothering to copy things from the ex's page in order to try and 'frame' her. I don't think anyone would really bother. I also can't see a stranger keeping this up for almost 2 months. A stranger killer would either be more blatant that they did something to the couple, or they would get bored and drop the social media and move on to their next victim(s). JMO.
My sights are on someone with an obvious motive. Having the mentality of 'you stole my husband' is a good motive, imo. Opportunity would be easy to come by as well. Heck, just invite them over because 'we need to clear the air about what happened'. They would probably go, without question. Could have been at a residence or even an out-of-the-way location. The suspicion probably wouldn't be there that something could be very wrong, because people tend to have a lot more trust in someone they know and care about. Their guard wouldn't be up. They'd just want to make peace.