I think they erred in stating that entry was forced into the locked room after medical transported Melinda.
From a safety standpoint for themselves and other responding officers, that makes zero sense.
If they knew there was a locked room that had not been cleared, they didn't know if there was an assailant still onscene holding SL and the baby hostage, they didn't know if SL was in there holding a gun to his own head and the baby hostage, etc.
It is police procedure 101 to completely clear the scene before ANY other action is taken, including seeing to the wounded, otherwise you run the risk of creating more victims.
Failure to clear the scene would mean that the first officers onscene put every single responding unit at risk, meaning other officers, medical personnel, and even SL and the baby. I can't see even the most inept officer with no sense of personal safety failing to do that.
Your scenario of them leaving SL in the locked room to keep him away from Melinda because they suspected him from the beginning means that they would have had to make up their minds on the way to the residence, before even discovering her, that he had shot someone himself. Before even talking to him to hear what had happened.
And even if that were the case, they wanted to keep him away and separated from her, they still would have wanted to make sure he was unarmed and that the baby wasn't injured as well.