No one I've read here so far has also accounted for the fact that the DB touched Dez as he was going down so when he made contact with the elbow and then his body before the ball hit, he should have been ruled down by contact.
It doesn't have to be accounted for; you don't understand the rule (not blaming you, the rule is stupid and confusing).
Imagine a scenario where a WR jumps up into the air to catch a ball. The ball is snagged between his mitts, and is not moving, while he is up in the air. As he comes down, still holding onto the ball, his tiptoes on both feet light scrape the ground (so that's two feet down). Before another split second passes, a DB delivers a crushing hit from behind and the ball comes loose.
That's not a catch. He has to maintain possession of the ball through a "football move" besides simply getting his feet down. It's incomplete.
But it's also not a fumble. Why not? He had the ball between his hands, his two feet touched the ground, and then he lost the ball, right? No. It's too quick, he never "secured the catch" and therefore you can't fumble it, because you never really secured possession. Possession did not occur with holding the ball and two feet down. It needs more than that... it needs this "football move."
Applying that to Dez's situation, he clearly had the ball in his hands (not moving). He then got two feet down. However, two feet down (or an elbow or knee for that matter), plus holding the ball still hasn't equaled a catch. It doesn't equal a catch until he makes a "football move."
So the defender can't tackle him yet, or touch him down by contact. He hasn't caught the ball yet, so he can't be touched down, not any more than a WR who is literally bobbling the football could be touched down. Dez has to "secure the catch" by making a "football move" before he can be ruled down.
Where the refs screwed this up was that they missed his football move -- Dez clearly dove towards the endzone. He did this after (1) grabbing the ball in the air, and (2) getting two feet down (maybe even 3 feet down). It still required (3) his "football move" though.
The refs saw it that Dez jumped up in the air, grabbed the ball, and then as he came down, his two feet touched the ground and then he fell to the ground all as part of the same motion. Thus, in their interpretation, it's no different than the WR whose two feet grazed the ground that I described above, if the ball is next jarred loose.
However, they were wrong. Dez's two feet touch the ground, and then he makes a dive for the endzone, springing almost without effort from when his two feet touch down, towards the endzone. It is a specific dive and stretch that you can see on the replay, and all during that time, he has secured the football. The dive and stretch with possession of the ball constitutes the (3) "football move," and at THAT point, he has secured the catch. Thus, when the ball is jarred loose when he hits the ground, it's either a fumble or down by contact (ground can't cause a fumble, etc).
They never got that far though. Under the rules, they needed to see a football move and they said they never saw one.
It's a stupid, convoluted rule, and it should be simplified so that it's much easier to see that what Dez did was a "football move."
But nonetheless your theory is incorrect under the existing rules.