How is it possible to marry a Western Muslim woman whose family is not Muslim, and to speak with her in a way that pleases Allah, knowing that her family has no guardianship over her marriage, and does the presence of one of her family members constitute a mahram for her, or must the meeting be in the presence of her Muslim guardian?

The matter here is easy. It is permissible to sit with the fiancée in the presence of her non-Muslim mahram (definition), as it is not a condition for the mahram to be Muslim. However, regarding the wilayah (guardianship) of marriage, a non-Muslim has no wilayah over a Muslim woman. If the Muslim woman does not have a Muslim guardian, the Muslim judge or his representative marries her. If neither is available, she appoints a Muslim man to handle her marriage.