So I think there are two big players in your experiments. Chocolate is complicated, and I won't pretend to know it all though. Chocolate has two big, arguably essential components: cocoa butter and cocoa solids. (White chocolate lacks cocoa solids, and is therefore a point of contention.) Melted fats are a liquid, water is a very different liquid.
As far as the liquid fat stuff goes, I'm paraphrasing Chocolate Alchemist to the extreme: Cocoa butter is actually a collection of fatty acids. Three are dominant and of particular interest: Oleic, Palmitic, and Stearic acids. These three collectively provide the crystalline structure of chocolate by connecting to each other, forming the cocoa butter crystal, and giving chocolate shine and snappy qualities. When chocolate melts, that structure is lost, the three go wild, and the reason tempering as a process is such a big deal is because those three acids will try to settle down at different temperatures. As a result, they won't return to the desired crystalline state without some kind of tempering process. (Which involves heating and cooling chocolate in a particular sequence, or agitating chocolate while it cools to prevent crystals from forming at all until all three acids are ready to cooperate, or using cocoa butter "seed" crystals that will encourage the other acids floating around to fall in line, into the proper crystalline structure... It's a terrifying process for a lot of baby chocolate geeks.)
So a lot is happening when you melt or cool cocoa butter chocolate, and similarly, adding any other fat will throw off the fairly delicate balance of fatty acids and drastically change the effectiveness of tempering and the texture of the final product. You should consider the melting temperature of any fat you decide to add at the very least - Coconut oil melts around 78F - so even without all the chemistry, it makes sense that if you add coconut oil to a fat solution that melts at 93F, you'll get something that melts at a temperature between 78F and 93F. When you dig into the chemistry of it, you find that you're adding a bunch of lauric acid and other shorter chain fatty acids that want to hook up with the cocoa butter and won't let the crystals form properly at all. Practically speaking, it won't set up and form a solid.
I once attempted to make a white chocolate ganache with coconut milk and...yeah, it never set up. It was velvety smooth, impossible to whip, never stopped dripping, and therefore completely useless to me. (It was a tasty sauce though). That was what led me to the Chocolate Alchemist once upon a time, and since real white chocolate was very expensive to me back then, I was sad. Hence my warning in the comment above. You can make something tasty with chocolate and coconut stuff, but it will be softer. It might not be what you'd call chocolate at all.
If you want to extend solid chocolate, you should look into paramount crystals - easy to use and give you a hard chocolate. Paramount crystals, or partially hydrogenated palm kernel oil, lecithin, and citric acid, are easy to use, raise the melting point of your chocolate, and contain an emulsifier to prevent separation issues. They're the main component of compound chocolate made with cocoa butter. Unfortunately, while they can be very helpful, they also don't contribute much to the aroma or taste of the chocolate, so I'd recommend using them sparingly. Some people absolutely despise the stuff lol. You could also look into edible waxes, but... again, use them sparingly. Wax has a very... waxy texture. Hard to describe any other way and extremely recognizable, and not generally desirable.
As for why you witnessed complete separation, my guess would simply be proportions, a resulting loss of emulsion, and the cocoa powder settling, even in your liquid fat.
Which brings us to the other feature you should consider in your experiments - the cocoa solids. These become more significant when you start adding water. The cocoa solids, not to mention the milk and sugar as applicable, are not liquefied in the chocolate, they are just extremely finely ground particles suspended in that cocoa butter crystal matrix. So when you melt and solidify chocolate carelessly, you may notice sugar or fat bloom on the chocolate. You may notice areas of dense, grainy chocolate next to smooth creamy bits. That's the result of cocoa solids and the other components settling out unevenly. My though is that by adding enough fat you saw this happen in an exaggerated sort of way, real time. It wasn't just the coconut oil refusing to mix, but the entire chocolatey emulsion breaking down.
As soon as you introduce water to the mix, as in some of your experiments, either in your mouth or in a cup of liquid, you dissolve any sugar and start to hydrate the cocoa solids and any milk. That hydrated chocolate mixture might remain thick and emulsified at first, especially if the solids aren't fully hydrated yet. At this point, you can add more fat and easily mix the two substances because those cocoa solids are holding onto the liquid for you. But the more liquid you add, the less dense the solution becomes, the less your cocoa powder can keep it locked up, and the harder it is to keep emulsified without additives like lecithin, and ultimately the cocoa powder will in fact sink to the bottom. Even if you thicken the liquid with corn starch for a drinking chocolate, the cocoa powder will settle out with a little time. And since you're dealing with a mixture that's mostly water at this point rather than fat/particulate, any added fat will act like... well oil to water. Even with added emulsifiers, it's not truly dissolved into a new solution. The liquid and fat never really become one, they're just held in suspension with each other, forming an often uneasy peace.
To summarize a bit, if you want to extend and dilute some solid chocolate, paramount crystals are your best, easiest option. You can use other fats with a higher melting point than chocolate, but even then, by doing so you'll interfere with the cocoa butter crystal structure and get something that while firm, likely lacks any snappiness. Cocoa butter is a fat that a lot of people think will work great in this application, but due to its chemical composition is arguably the worst lol. And many of us learn this the hard way.
When you add water in any form to chocolate, you've created a whole new liquid emulsion to which different rules apply. While you can make a partially hydrated kind of ganache that sets up firm, the particulate can only absorb so much liquid, and you can only add so much fat to any emulsion before it breaks down.
