I use a box grater because it makes the carrots a lot less wet than grating them in the food processor does. If you use a food processor or other electric appliance instead, I would lay the carrots out on a towel dry them afterwards so you don't end up adding extra moisture to the batter. 

Some of my favorite fall vegetables and fruits are Brussels sprouts, butternut squash, cauliflower, sweet potatoes, and good, crisp apples.

I think the basic vinegars to have on hand are white wine and/or champagne, red wine and balsamic. I also use raspberry, apple cider, and sherry vinegars occasionally. 

The only way to learn to use a pastry bag is to practice. I would make a big bowl of whipped cream fill a pastry bag, and pipe the whipped cream onto a cutting board. When you're done, scoop it back into the bag, and try again until you have the hang of it! 

I haven't tried substituting anything for the wine, but because the recipe calls for such a small amount, I would think you could substitute apple cider - or even water!

No, I would not recommend freezing the Apple Pie Bars. They are best served fresh.