Ok, an article written about Gideon's death was dated June 16th, and it says he was shot that Sunday. Unless they played loose with the timeline, Mobley discovered Romero sometime around then.
At that point, Elliot had already been in jail for about a month. Romero's body looked way too fresh to have been lying there the whole time.
I went back and revisited the scene where they show Romero's dead body and you are right he doesn't look dead for more than a couple of days, meaning that Elliot couldn't have killed him if like you say they didn't play loosely with the timeline which I don't think they did.