My thoughts on this are a little different than yours.
I found it odd that suddenly once the video stills appeared, so did this whole online conversation, and not before.
I think the RSO subject may have come up with the SIL and her family, possibly the weekend before,everyone was upset, she told Brandon and possibly said her family thought she should distance herself from him somewhat even though she believed him that he didn't do it.
I think Brandon lost all of his coping mechanisms he had learned in prison and tutored others on,went out, impulse took over, he needed the instant gratification and feeling of being in control, and Mickey disappeared.
There is still tension in the family all week, they don't trust Brandon, and then on Friday 2 stills are released, the Shuniks tell eveyone that the white truck pic released is NOT the one they are looking for but there IS another white truck. The family then confronts the young woman telling her they think Brandon may be the perp and a ruckus breaks out. The next day, brother-in-law goes online,starts talking about this strange dude in the event that it is him. By doing this he will have covered for the family and let others know they were suspicious of Brandon so they don't look like fools to others in the event he is the perp.
MOO.....