One reason (and one that would be a reason you are so forcefully pushing a narrative that makes the LEOs look bad)
is that there was more than one shooter.
It’s also possible those items were discarded as they fled the scene - to tie in with the Nun on the Run in the purple van story.
Another reason is those items could have covered up the guns and were tossed aside by Lanza. He could have opened all of those doors.
Or the witness could by lying and the doors weren’t open.
There are a number of reasons that don’t require the LEOs to do warrantless search on an immobile car not going anywhere where they contaminate the crime scene.
So the nuns disrobed after they got out of the car?? What sisterhood order wears black sweatshirts??? the order of the black hoodie???