The complex they lived at was really pretty upscale on the way out of St. Pete to the causeway to Tampa. Terri's brother Bobby lived there too but in another building. There are lots of bldgs, some gazebos and a curvy parking lot. I have no idea if Mikey acted alone but it took him a long time to do anything. Then Bobby showed up to see what the heck was going on.
That makes me think, even more, that Mikey may have had extra keys made.
One of those visitors (or a friend of the visitor) could have let themselves into Terri's place that night. Maybe it was arranged for Michael to be at work when that person confronted Terri.
Michael may have been in shock, when he arrived home and he realized she was injured, yet alive. He would have needed extra time to think over his alibi.
I wonder if it is possible that Terri's "collapse" lasted 6 hours or more.
Michael may have been threatened by the fact that Terri was gaining confidence in her attractiveness. Maybe he was hoping to make her jealous of him, by subtly letting her know other women were willing to have affairs with him.