Hey, so this will be my first year that I participate in the costume contest. Before I register I wanted to know if my silicone mask puts me into the professional category. It is the only piece that could be considered professionally made, as I am DIY'ing the rest and am by no means a professional.
Did you make it yourself, or did you buy it? If you bought it, was it custom made, or "store bought"? They're not going to force you into the professional category unless they look at your costume as a whole and say "this is professionally made". I did the contest last year and although most of my costume was hand-made, I did incorporate some leatherwork that was made by pros, but it was a minority of whole, and I didn't have to register as a pro.
PS -- Sign up in your preferred category. They have to "pre-judge" you first, and if they determine that you belong in the pro category (and it certainly doesn't sound like that's the case) then they'll switch you.
Keep in mind GenCon is primarily a craftsmanship contest with a touch of stage performance thrown in. Emphasize the parts you made during prejudging, and you should be fine. besides, if they decide that you should be in professional category they'll tell you and move you to it, they won't disqualify you.