I think he immediately - and probably sincerely - decided she was guilty and was disappointed no taillight evidence was at the scene. Dude clearly has the IQ of a mentally disabled gorilla.
By moving things along by planting evidence he was helping both his friends the Alberts and himself. Make it a slam dunk so he didn't have to do any serious work, like identify and interview all those witnesses. So much easier to just do a group chat with some of the principals and call it a day. Clearly he had no worries that anyone would question his useless investigation, including things like taillight patterns and the origins of broken glass.
The state police system in MA needs a complete overhaul and it should start with removing anyone who had any oversight responsibilities for Proctor.
Looks like he initially thought it was as a result of a fight.
In a text chain, Proctor stated that 'his initial thought was that this was as a result of a fight' (or similar wording). In the same text chain the respondent replied 'that I bet the homeowner will get some serious sh.'. "no because he is a Boston cop'.
Despite being his inital suspicion. there was no mention of anything about a fight anywhere in the police reports. Nope, nada.