The split is necessary. Just as necessary as existence-consciousness-bliss [...] Without it, realizing what we have realized is impossible.

Yes. It's the "it reflects itself back at itself" in my previous reply. However --

Existence itself wouldn't exist without it

Existence exists happily without twisting itself into self-reflection. But even talking about existence as existing or non-existing is rather imprecise and sort of redundant. Existence, aka Being, has no negation. You can negate it only in your mind, but that's it. The existence of negation as anything but concept in one's mind is illusion.

as conscious existence infinitely experiences bliss.

Experience is necessary neither for existence nor for knowing or bliss.

The split allows existence to infinitely become more blissful!

What increases is experienced happiness, aka manifestation, aka perceivable emotion. The underlying bliss (aka that which manifests as the perceived fullness of experience) is at all times the same. The same is true for existence and consciousness (after all we are talking about the same). None of them increases or decreases. They simply are. Realization is then an "on/off switch", and it simply reveals that everything is that.

What is your theory on what and why is the split?

The split merely allows self-reflection and self-interaction, but I don't think that that is its "raison d'etre" (it seems that there is none). Self-reflection and self-interaction seem more like an effect. And as for what the split is (except that it is essentially not there), I have no clue. Hence, my speculation ;)