Is this 100% dextrose? Is it powdered? Or granulated like sugar?

Thank you for your inquiry. Yes, the corn sugar is 100% dextrose. It is a powder form.

Is this used for back sweetening ?

According to internet a sugar is not preferred, using the juice you used to make your wine is preferred and just a little. I have never made a dry wine that needed to be back sweetened, it would work in a pinch, but the juice is better.

How does corn sugar react with Yeast ? Will I get the same of alcohol amount as with sugar?

It definitely reacts well with the corn sugar as far as alcohol content and I feel that the corn sugar makes my wine taste better than using cane sugar