I still wouldn’t take the Winnower and the Gardener as actual characters.
I (emphasis “I”) see them more as the personification of the concepts of the Light and the Darkness.
As in the light brings up complex life and helps it flourish (gardener).
The darkness reaps beings and gains power from said reaping (winnower).
I think the entity like the traveler are the closest thing to actual personifications of those cosmic powers, they are not the light or the darkness itself but the closest thing we have to a “gardener” and a “winnower”.
Mind you the Darkness isn’t evil and the light isn’t good. Beyond Lights campaign is about using Darkness as a tool NOT a philosophy. We see the light as almost a religion not just a tool (I think this will change with WQ).
Tl;dr I like your theory, I still believe that the entity and with that the traveler are the beings that use their respective power at maximum potential and follow it’s associated philosophy zealously.
The Winnower itself refers to the Traveler as the Gardner. The only entity in existence that we know of that actually knows the Gardner.
It’s also in the way it speaks. It puts the actions of the Traveler on the Gardner as if she did it.
You wouldn’t do that if it was an agent. I think I know where you are coming from, D1 lore had that in the BoS but Unveiling supersedes D1 lore.