There are different views on just what percentage to pull, and even the method to use for whisky distillation. Most will use pot distillation for whisky production, and pull an average finished product in the range of 60-65% ,with pot distillation the percentage will start higher, and gradually decrease,. Some are starting to use reflux distillation in order to gain better control, but most traditionalists would not hear of such a thing. In answer to your other questions:

1, I would not consider corn sugar to be a grain mash. There are some, even distilleries, that would call this a grain mash, but most would disagree. I have heard, however, of people making a reasonable whisky from this.

2, The difference between whisky and vodka is not as simple as 80% vs. 95%. Vodka by definition is colorless, flavorless, and odorless. There are distilleries using exclusively pot distillation, and never achieving 90%, that produce vodka. There are rums that are pulled as high as 90%. The real factors that determine the resulting product are the base that the mash was made from ,due to the flavor contribution,, the distillate collected, and any oak aging or carbon filtering that may be involved. There are vodkas being made from rye mashes, which you would normally think of for whisky. A different distillate collection, remove the carbon filtration, and age in oak, and this would be a Rye Whisky.

3, If you want control, then you should use reflux distillation. This will allow you to collect individual boiling points until you have a mixed product in the %abv that you are looking for. I would suggest going by smell or taste, though, and not based on a goal of a certain finished alcohol content in the distillate.