Here is my total guess at this:
Ehrlick is a drug user.
Ehrlick knows the Wallis guy and has access to his home as a result.
Ehrlick takes the Suburban because he either needs it to score or to do something he has to do to make the money to score.
He has Robert while mom is at work and is either high or in need of being high, we already know Robert was supposedly grounded and being punished, he kills him intentionally or not. Cleans up the scene, returns the vehicle so the stolen vehicle (which he is probably hoping hasn't been reported yet) so that doesn't come back to him. He knows they have a "missing" kid and a bio dad to deal with so the police are going to be around.
Wallis sees his vehicle is missing, knows Ehrlick has been around, comes back and his vehicle is back and Ehrlick's girlfriend's kid is all over the news as missing. Wallis reports his vehicle as returned and tells the police of the connection to the family.
Ehrlick suddenly becomes suicidal and ODs.
Forensics will place Ehrlick and Robert in the vehicle.
Hopefully they will also place the vehicle at the site where the think the body was put in the river.
Ehrlich had his own vehicle to "score" with. Why would he draw Wallis into it? Also, the neighbor notified Wallis about his vehicle being "missing". At least, that's the "story".