This doesn't really have anything to do with saying that people who don't accept the Bahia's prophet as the Messanger for the current age, cannot be spiritual, as you said. Are you not planning to defend that anymore, since I've shown how that's not valid to say that?
A changed heart and mind will however, and meditation and prayer are tools for just that.
How long ago was that? The world is evolving at an enormous pace, and social programs 150 years out of date, may not be adequate. As far as spiritual pathways, those tend to be timeless, and there have been and are plenty of guides for that already, from thousands of years ago, to days ago. I personally find the idea of only one voice every 1000 years, to be a rather limited perspective. I think new voices can be found every day, coming from those who have developed spiritually enough.
We try to do that today in our progressive and liberal institutions. But conservatives prefer tyrants and power over minorities as the godly way, it seems.
It also takes voting to stem the tide backwards towards enslavement.
Anyway, my original point was that someone does not need to accept the Baha'i prophet in the way the Bahai'is do to be spiritual, as you said, anymore than you need to accept Jesus is the 2nd person in the Trinity to be spiritual. Those are religious beliefs, and spirituality is not dependent on those. Spirituality is innate to all humans, and does not come from prophets outside of ourselves, or teachers, or anyone else. They may help point the way to find that in ourselves, but they aren't necessary for spirituality. SBNR is proof of that.