I get the same error message
I don't know if there's anything you can do, sorry. I've spent some time googling "dng file repair", as I'm sure you've done yourself, but there don't seem to be any good solutions leaping out at me.
With respect, that's impossible. All DNG files from one camera should be a very similar size. Please check again. It sounds like you were looking at jpeg files.
And? Have you compared it to other DNG files from your camera? Is 20MB correct?
By the way, you're in Adobe RGB. This is disastrous. Fix it right away. Click on the blue link at the bottom of the ACR window, and change from Adobe RGB to sRGB.
No. All this "looking" stuff is feeble and irrelevant. The only way to judge colours is against pro lab prints. Do you have some pro lab prints there?
Furthermore, what program are you viewing in? If it's not an Adobe program, forget it. Only view in Bridge.