The white is a fault, but I don't think it would prevent you from finishing her. Depends on what the competition is like in your area, how she ultimately matures, and the judge for the day. She's definitely a working type dog, so you may do better under all breed judges. Presentation will be a big part of showing her, I've seen some fairly ugly (IMO) working dogs win over dogs with more of a show type, because they free stacked, gaited well, and the other dogs just weren't shown to their best advantage.
As far as registration goes, I'd get her AKC registered. You live in the US, and at some point even if you don't do conformation you may want to put some obedience titles on her. Start the process now, and then later down the road you won't have to worry about it, it will be done. Besides which, AKC has changed the requirements for registering Malinois (and other Belgians) on more than on occasion. I'd just get it done, that way if they decide at a later date to make changes again, you won't be effected.