Is it permissible to sit with one's fiancé while wearing the niqab and in the presence of mahrams, knowing that the conversation revolves around matters of religion and marriage?

There is no harm in the fiancée speaking with her fiancé, who is a non-mahram (definition) to her, to arrange matters of the marriage and related affairs, provided her mahram is present. However, this should not be excessive, nor should she be present at every gathering he attends when visiting her family. Her presence should be limited to gatherings concerning the arrangement of marriage matters where there is a need for her presence. If her guardian suffices for this, it is more appropriate.