OP,
I think your scenario is possible. I would ask which LE agency the bad cops belong to? Newtown, State, or something else?

There is a radio recording where a cop calls dispatch for a check on a Connecticut license plate 872-YEO, saying it's a possible suspects vehicle. Dispatch responds that it's registered to Christopher Rodia with date of birth August ? 1969. He was arrested in July 2012. Maybe his car was seized?
[link to www.westportnow.com]

His car was photographed as it was being towed from the Sandy Hook crime scene. Scroll down this page for photo.
[link to spitfirelist.com]

Is it possible that the 2 bad cops drove to the Lanza neighborhood in separate cars, left cop car a block or two away from Lanzas home, then both cops took Rodias car to the house. That way they would attract less attention.

Once there they shot Nancy, drugged Adam with scopaline, dressed him in camo pants and whatever else. Grabbed all the weapons and headed back to the school picking up their unmarked cop car along the way.

Maybe these 2 bad cops were plain clothes, not uniformed? Just a shiny badge and black armored vests. That way there would be less costume changing in the midst of chaos? Maybe they were members of some task force with ATF?

I can't find the the link for the radio scanner. It may be in here somewhere.
[link to www.youtube.com]