It’s not about justifying his death.
It’s that the DA is prosecuting the cops on the grounds that the arrest was unreasonable/illegal; this new evidence further erodes that ill-advised approach.
No, it doesn't. If the officers didn't see him drop anything in the building, and didn't recover anything he dropped from inside the building, then him entering and exiting the building is wholly irrelevant to the legality of his arrest. Especially since the arrest paperwork does not mention anything about his entry into the building.