Congratulations on going. I have never been, but have talked with others who have and have read people's feedback from prior years. Apparently if you have preview night, you have a good chance of getting dolls. If not, and if Mattel don't fix last year's issues with the booth opening times, then it may still be difficult. Hopefully they do fix it, though.
It would have been great to know a long time ago and you could have tried for the pre-sale voucher, but it sounds like you just found out.
If you don't feel like getting in line at 5, 6, or 7am then you might want to purchase from my friend who is going to the show. He has sold his pre-sale vouchers, and is only taking reservations from people for dolls he gets at the show, but the premium he charges is reasonable and it may still be worth it for you. Message me if you want more information.