Sorry to hear about your fiance, and oral swab is as good as any other aproveed test, the main issue is the time of the test, you need to have 3 month after exposure test to have a conclusive negative result, regarding the child question, an Hiv postive father can have negative children, there is a technic called sperm washing, im not familiar with, but you should contact and Hiv specialist to get accurate information about this.