:salam2:
i am very pleased whenever i hear someone embracing islam. but i have read some things in your post that are of concern to me. the first thing is that the bf/gf relationship is haram in islam. the second thing is why would a non-muslim wear niqab? i know that people have already told you about the whole girlfriend thing as being haram, but they really haven't explained why it's haram.
firstly, when one embraces islam it erases all of one's previous sins. alhamdulillah for that. but if you continue in a practice such as dating and quite possibly comitting zinaa ( fornication ) you aren't starting off on the right foot. you embrace islam to save your soul from the hellfire, not to put oneself into it. islam is about willful submission, submission to the creator; therefore we have to give up some things or some people as part of the submission process. we as muslims must believe that everything that ALLAH SWT made lawful is of benefit to us and whatever ALLAH SWT has made unlawful to us isn't of benefit to us. so what i suggest is that you either marry this girl as soon as possible or gently let her go. i say this because it's impossible to be a pious muslim while maintaining an unlawful relationship.
her wearing hijab while not having said shahadah is of no benefit to her. she may have great respect for islam, but if she doesn't say
with sincerity before she dies ( which can be at any time ) hijab and niqab will not be of benefit to her. because she is not blood related to you, she shouldn't even really be alone with you. i hate to seem harsh but if you want to practice islam, you have to practice it properly. have her talk with some sisters at the masjid, if she isn't able to attend, she should call 877-why-islam. they have live volunteers to answer any questions that she may have about islam. then she can truly check her heart and see if islam as a way of life is for her. and if she does embrace islam, she has to adjust herself by limiting her contact with you or she has to marry you as soon as possible. if she isn't ready for marriage, she may have to leave you, so that no further fitna occurs.
my hope is that she embraces islam and you marry soon.