Srila B V Narayana Maharaj is one of the seniormost Gaudiya Vaishnavas on the planet. I know there are a lot of misleading statements made both ways, by devotees either from ISKCON or GM.
We should learn to see our own shortcomings, instead of "prying on the frailties of others". I have found the books of Shrila Narayan Maharaj very enlightening,as compared with books published by recent ISKCON writers other than Shrila A C Bhakti Vedanta Swami Prabhupada.
Yes, Srila Narayan Maharaj is not an ex-ISKCON Guru. A Guru is not a part of some organization or institution. Anybody who is following the teachings of His Guru parampara can be a a Guru.
About marrying someone who is disciple of another "Guru" : first of all, you both must be at least kanishtha adhikari. Then you will not see such differences between "Gurus" and realize that the same Guru tattva manifests everywhere.
However, if you find a conflict at the spiritual level, it may be better to follow one Guru than two. Conflicts at a spiritual level are much more difficult to solve than those at mundane levels.
Hare Krishna.
(neither ISKCON, nor Narayan Maharaj follower, I am a Gaudiya Math disciple of Shrimad Hrishikesh Goswami Maharaj, disciple of Shrila BSST Prabhupada).