Distinguishing clerics, druids, and warlocks?

What is the difference between a druid and a cleric devoted to a god of nature or a warlock who made a pact with a god of nature? Aside from their game mechanics. What is the fluff and world building distinction?

The official D&D settings do not have satisfactory explanations. The world building of D&D settings in general falls apart upon inspection. So it falls to homebrew and 3pp to impose a sense of logic to the often bizarre rules of D&D and its derivatives.

Clerics, druids and warlocks are based on fantasy archetypes that predate the game itself. Clerics are originally based on vampire hunters. Druids are loosely based on the druids of the lost Celtic faith. Warlocks are originally based on witches as described in the Malleus Maleficarum. When they became classes in the game, their backstories and capabilities expanded and evolved over time.

Although their mechanics are distinct, their fluff overlaps. Perhaps the easiest way to solve this is to remove the overlap. Do not allow them to worship or make pacts with the same deity/patron (although fallen deities might be suitable as patrons). Instead of their being evil gods worshiped by clerics, there are not strictly speaking evil "gods" but archfiends and so forth. They are represented by warlocks, not clerics. (Although this requires actively defying the D&D convention of evil gods, which I don't have a problem with since I find the concept as it appears in fantasy gaming rather silly. Golarion, for example, has entire nation-states that openly worship evil gods and based their morality around being evil like the villain in a children's cartoon.)

Several campaign settings offer the explanation that clerics and druids worship the new and old gods, respectively. These settings include Scarred Lands, Kingdoms of Legend, and Points of Light. In these settings, there was an ancient war between gods loosely based on the Greek titanomachy. The druids venerate the primordial gods/spirits of the natural world, whereas the clerics venerate the new/orderly gods of mortal affairs.

As warlocks were not OGL until fifth edition, they are not integrated into most campaign settings. So no help can be found there. The existence of warlocks requires drawing a line between deities and patrons. What exactly is the cutoff point? What makes a god? The answer depends on the predilections of the world builder, so you can't simply use the one-size-fits-all approach of the rules.

An idea I had was to bring back the whole evil warlock shtick from medieval times. Gods, primal spirits, etc can freely hand out power for whatever reason. But fiends, fairies, or what have you are limited to pacts with warlocks for whatever reason. Perhaps they simply lack sufficient power, the freedom to channel power, whatever. (I suppose this sort of approach would fit better with a Eberron-esque agnostic approach to deities, which would better distinguish clerics and druids from warlocks. The logic being that deities are questionably definable whereas patrons are observably real entities, so their application as power sources would be different.)

Since demons/devils/fiends/whatever wouldn't have all that much power in the world because it has built-in defenses (you know, whatever prevents demons from just opening portals willy-nilly and invading the world), they need to rely on warlocks. They make pacts, buy the warlock's soul, grant bits of knowledge or power to allow the warlock to operate without wasting time on becoming a wizard. (There's conceptual overlap between the warlock and sorcerer due to their use of Charisma, but I'll ignore that for now.)

At this point, you'd need to world build your demons and other patrons. Again, I don't like the game's one-size-fits-all approach. For example, a lot of demons in mythology don't live in the underworld or hell; indeed, the mythological definition of demons is far less rigid than D&D's. I'd have to reserve that for another post, though.