That both engines were operational is a poor assumption, you have only one factor of 3 indicating it was, and that factor is manageable.
It's a reasonable one, though. Here is the evidence we have from photographs and about the plane:
1) The plane was right side up on impact. It did not Vmc roll.
2) The damage was "limited" to the front of the fuselage.
3) The plane was losing speed and altitude.
4) The weight on the wings is in the neighborhood of 17 lbs. per square foot.
From this we can form a hypothesis. For example, we can hypothesize that the pilot made great attempts to keep the plane's attitude level. For example, when he was losing altitude he nosed the plane up and made a decision that sacrificed speed for altitude. Why he did that is an interesting question. But the data suggests that he didn't (or couldn't) add power.
Let's assume he had an engine failure. If that's the case then the one thing you don't do is climb. You level the plane, stay away from red line, go full power with the good engine, identify, verify, feather. We don't have evidence that an engine was feathered. Further, a single engine, IFR approach to minimums would favor a decision for a gear up landing. It's a huge and very well lit runway and while the plane was slow, it was close enough that a clean plane on one engine possibly could have made it.
But he clearly chose not to do this and instead chose to turn left while continuing to lose altitude and likely speed. The latter is drawn from the evidence of how much of the plane was in tact on impact. Moreover, there was no Vmc roll - which given the speed of the aircraft would likely have been present with an engine out.
So, given the decisions the pilot seems to have made, the disposition of the plane on impact and the other evidence currently available, an engine failure doesn't seem to fit. It's certainly still possible. But for me there's too much evidence inconsistent with that hypothesis.
Please note, it is not lost on me that we are talking about the deaths of real people. People, it should be added, who have loved ones who are grieving their loss. My conjecture here is in no way meant to minimize or trivialize their heartbreak. I'm not doing this as a game. Rather, my hope is that by having this discussion that those of us who pilot these machines can learn from what happened and, perhaps, avoid a future loss.