How to mix chocolate into flourless ground almond cake?

The typical way for this kind of recipe is to fold the melted chocolate into the beaten eggs. Make sure you are really folding it, not stirring vigorously or beating with a mixer, to ensure minimal loss of volume. When the chocolate is semi-incorporated, continue adding the almond flour in batches.

The resulting cake will be denser and less dry, but that is normal for chocolate cake.

open the door to being able to combine many other ingredients (and, thus, produce many other variations on this cake.)

I am afraid it doesn't work that way. What you propose are not some kind of minor variations (those would be e.g. adding a few drops of aroma, or a food coloring, or baking it square instead of round). If you start adding main ingredients, you will not only get a very different texture, you will have to change the rest of the process too - basically, you would have to create different recipes. Not impossible, but it is a huge amount of work, and it is much easier to just use existing recipes.

For example, the addition method I described above is specific to melted chocolate. If you wanted to add, say, sour cream, or a fruit puree, you would have to interleave it with adding the flour, and start with flour - so 1/3 flour, 1/3 puree, 1/3 flour, etc. But the almond flour is also not absorbent enough, so you would have to experiment with different flour combinations until you get something that works well, and you like the taste. You can do that of course, but the more pragmatical way is to just follow an existing recipe, instead of creating a new one out of nothing, when you are not an experienced baker already.