This is how I had seen it explained. Leo is the sign of the self, the arts and acting (among other things). Neptune is the planet of selflessness (selflessness of the self in Leo?), inspiration (every artist needs inspiration) and illusion (every actor needs to be able to maintain an illusion of the self).
This would place Aquarius as its fall. Aquarius rules commune style living, friendship and radical change. Neptune rules solitude (which would be the antithesis of communal living and friendship) and transcendence of the spirit (change yes, but a very different kind of change).
Ok, for some reason I wasn't able to see this post before.
Please, don't feel like I am attacking your ideas or sources (if you can help us out with providing them?), just that...Leo as the sign of the self? Really? Leo is the domicile of the Sun, but it isn't the sign of the self, because the sign of the individual self involves a process that is both complicated and yet simple, and includes the planets, the angles, and a host of other things specific to any chart. Also, I'm pretty sure that even modern astrologers give the Venus signs, or Venus herself, to the arts, and acting.
Neptune is what? I've seen it as delusion, illusion, or even the...mists? I haven't seen it as the planet of selflessness...again, and only because I really am interested in all opinions/ideas, do you have a reference for this idea?
Ok, so now we are at Neptune, the planet of selflessness, being exalted in the sign of selfishness....why? Because being selfish works better for the native if it is selfless? And further, if it is exalted in the domicile of the Sun, does the Sun then exalt Neptune? Of all the planets? Further...if Neptune is then in fall in Aquarius (since the sign opposite exaltation is fall) why is the planet according to these sources that represents selflessness in it's fall/depression in the sign that modernly is represented as the sign of being concerned with the greater good? And, if Neptune is in fall in the sign of the Sun's detriment (and that has a relationship to Saturn) does this actually equate Neptune to the level of the Sun?