Does this technically make Thelema and Theosophy parallel ideologies, like Pantheism and Humanism?
I will admit that I am fairly under read when it comes to Theosophy, but from what I can gather about The Secret Doctrine it seems to me like its concepts of spiritually-guided evolution are at least superficially similar to Will.
I think you're right about cosmology. Crowley himself seems to be rather coy about his metaphysical ontology, if not outright dismissive of the discipline, which seems to the bulk of what Theosophy focuses on. I feel like some of the self-identifying Thelemites I've spoken to have been influenced by Theosophical ideas without realizing it. I'll admit that I may have been one of said Thelemites.
I'm not sure, however, whether there are any outright contradictory teachings between the two. Due to their radically different focuses, I do sort of doubt that such contradictions exist outside of maybe an exoteric understanding of correspondences.
A somewhat related question is, can Theosophic cosmology run in tandem with Thelemic practice? In that sense, we would be playing to the strengths of both disciplines and allowing ourselves a bit more wiggle room with some of the potential conflicts. Would such a practice damage your integrity as either a Theosophist or a Thelemite?
I find it hard to believe that organizations of either sentiment would be too opposed to the mixing, as they tend to be rather open-minded and, again, I find plenty of Thelemites flirting with Theosophy as it is. I think any real answer to this new question would be a rather subjective one, but I am interested in opinion.
Well things can definitely get blurry in places.
For Blavatsky we have more of a closer connection to the traditional Dharmic cyclical view of time, where things go back and forth between ages of a certain manner - yet we have the three Thelemic aeons which have a more progressive view of humanity transmutating through quite distinct and intensely contrasting phases of consciousness. (Ma'at presumably after Horus).
We've also got the quasi-supernatural (but definitely metaphysical) concepts of Mahatmas (comparable to Bodhisattvas intentionally), and Crowley with Secret Chiefs and later the HGA (Holy Guardian Angel).
Remembering also that in chapter 30 of Magick without Tears Crowley writes in the postscript:
"My observation of the Universe convinces me that there are beings of intelligence and power of a far higher quality than anything we can conceive of as human; that they are not necessarily based on the cerebral and nervous structures that we know; and that the one and only chance for mankind to advance as a whole is for individuals to make contact with such Beings."
These things do have seblence but not identical.
What you say about the Will is very interesting because in Thelema as explicated by Crowley himself
(maybe not necessarily the run-of-the-mill person that calls themselves a "Thelemite" though) and most eloquently in the Book of Lies - that The Will, the divine Will, the Thelema, is the course and drive of all things, the microcosm and the macrocosm. We are ourselves literally incarnations of that Will. Our purpose in life is to connect our consciousness to that Will through spiritual practices to attain alignment with the center of our being.
Theosophy doesn't have anything quite as direct but still bases it's views upon quite Qabbalistic framework like Crowley, both of which going through Kether down to Malkhut (regarding the emanationist side of metaphysics).
As far as both be fused together? yes definitely I think so. On some level, Kenneth Grant already started doing that with his Typhonian tradition (though that also heavily focused on other aspects but payed very due homage to Blavatsky's importance).
As you already say, many people would already be fusing Theosophical ideas with Thelema already, and in my experience in various occult communities, I've already come across people that are devoted to both in one form or another.
I've still yet to really explicate exactly what I want to say on the matter (which can be quite vast), which I will do as soon as I bump into that headspace, though I've written many notes on this stuff over the years that I should revisit.
- I wonder if Alice Bailey's work corresponds here as well.