I see you are following the Ken Wilber's model of the states of
consciousness.
When I think of Gross or Nature Mysticism, what comes to mind is
the experiences of total unity without inside and outside with
regards to nature.
That means in a natural setting or environment, while looking at
a tree, the sunrise, etc... suddenly the sense of an inner-outer
division dissolve and for a moment you BECOME the tree, the seeing,
the sunrise, the environment. It is a temporary non-dual experience
with relation to gross forms. However it is not non-dual
insight.
When I think of Subtle mysticism, what comes to mind is the
Vajrayana practice of visualizing and attaining union with a deity.
Unlike gross mysticism, I do not have experience with this because
I do not practice Vajrayana.
When I think of Causal or formless mysticism, you are right that
what comes to mind is the practice of self inquiry ala Ramana
Maharshi style. This is emphasized in Advaita, but also in some
schools of Buddhism, some form of Zen emphasize this like Ch'an
Master Hsu Yun's teaching. It leads to the I AM/Witness
realisation.
When I think of Non-Dual mysticism, I think of some kind of
inquiry into the relationship of presence/awareness and phenomena,
or some kind of koan, that leads to the realisation of the unity of
absolute and relative. This results in non-dual realisation.
Similarly, Vipassana practice leads to non-dual realisation at the
later phase of one's practice (Anagami-hood and Arhatship)
One *does not* need to climb the above in linear stages. First
of all, gross mysticism and subtle mysticism, the resultant
experience cannot be considered a form of realisation or
enlightenment. But the Causal realisation of the I AM is a form of
realisation as there is a direct insight into luminosity and one's
true identity. Similarly, Non Dual is a form of
realisation/enlightenment. Therefore the I AM and Non Dual are
listed in
Thusness/PasserBy's Seven Stages of Enlightenment
You *do not* have to go through gross mysticism, though from my
experience by simply practicing mindfulness it is likely you will
have glimpses of unity in a natural setting similar to gross
mysticism. You *do not* have to go through subtle mysticism to get
enlightenment either, as many Buddhist teachings do not go through
visualisation on deity, etc as well.
And you even *do not* have to go through the Causal level of
realisation. Though many teachings like those of Advaita actually
goes through the Causal level first. Our moderators Thusness and
Longchen went through the Causal level first. I, too, am going
through this level first and I am focusing on self-inquiry.
However this is not the case for everyone: there are in fact
practitioners who skipped the causal level of realisation and gone
straight to Non Dual realisation. For example Vipassana
practitioners like Daniel Ingram who actually reached non dual
realisation and the realisation of anatta without having gone
through the I AM phase.
However the Gross Mysticism, Subtle Mysticism, Causal Mysticism
can be helpful parts of the path and can bring some important
insights. For example, having insight into I AM will make it faster
for other insights to come as one has direct insight into one's
essence/luminosity... that is why it is called 'direct path'
whereas Vipassana is considered gradual, because the insight into
luminosity comes only at a later stage in that path. This however
does not mean Vipassana is any less effective, it can still lead to
enlightenment at a very fast pace. Also someone who has gone
through I AM then Non Dual may later come to value Vipassana
practice as well.
That said, Non-Dual is still not the end stage of realisation.
Non Dual is only Stage 4 in
Thusness/PasserBy's Seven Stages of Enlightenment. There is
still the realisation of Anatta (Stage 5) and the realisation of
Emptiness (Stage 6) - so if you follow Ken Wilber's model, you
still have not reached Buddhist enlightenment. Ken Wilber is still
at Stage 4 in
Thusness/PasserBy's Seven Stages of Enlightenment.