Since the killings happened less than 5 minutes later, they're going to have to claim that Alex left at that very moment to go back to the house, and that he somehow managed not to hear any of the shootings. And then that he left the house some 10 or 15 minutes later from that, which is also when he was then trying to call Maggie and text her without a response, but didn't think to simply go to the kennels to find her where he would have just left her. Not to mention also needing to explain why he told such humongous lies about his whereabouts to the police (and to his friends and family).
The last witness Nathan also brought up some bad points against Alex regarding the kennels. He said that they would have turned on the lights out there when it was dark. Apparently the lights were not on when Alex supposedly found them. Meaning an outside killer would have had to turn off the lights for some reason.
Defense have also been trying to say that the water on the ground was from them washing out the kennels at that time. But Nathan also testified that when they would wash out the kennels they would put the dog beds on top of the cages so they don't get wet. I'm sure the beds were not found on any cages that night. (Their defensiveness about this also convinces me that Alex did indeed wash something off out there.)
To be able to get in any explanation for all of that, I see no other way than for Alex to testify. But I don't see how that could be anything other than a disaster for the defense. Even if he managed to use his vaunted people skills on the stand, the jury has heard lots of testimony about how he has lied to his closest friends and family and fooled them for years and years, so whatever he says should get a lot of side eye.
To me, as soon as they got the cell phone timeline data in on when the phones go inactive combined with his voice being identified 100% on the kennel video, the case was over. Nothing other than his guilt explains that. Everything else is superfluous.