JMO we have all seen cases where an abused person gets a light sentence and generally, only women. For myself, I try to keep an open mind that SM could have been an abused spouse at this point, and possibly was being threatened with his own life. I don't know if this is the case, but it does not seem out of the realm of possibility. However he seems to have certainly lied to police to cover up, etc.
Anyway, I have trouble seeing a motive on SM's part if he was not under some sort of threat to himself, or maybe even to children. TM seems like she would stop at nothing. Not trying to give him the benefit of the doubt, or defending him, but I am not convinced he participated as eagerly, shall we say, as perhaps TM did.
Also not convinced he knows where Heather is, as he does not appear strong enough to withhold much for long.
JMO