I have a really hard time wrapping my brain around these concepts. They feel so abstract to me.

It is hard to explain without the equations. Let me try.

You have fields that have specific properties under the interactions of the theory. For the Standard Model particles, this is how they interact electromagnetically, weakly, strongly. For the dark fields, this is how they interact darkly. Every single possible interaction that is not forbidden by a symmetry is there. That is the general rule. And this also applies to masses.

Then, you can extract the mass matrix associated with the different field. This matrix can have diagonal and off-diagonal terms (if allowed by the symmetry, such terms are the nixing terms).

But the physics objects, those we observe, are those for which the mass matrix is diagonal. Therefore, the physics objects are admixture of the initial ones.

Is this clearer? I am not too sure

Dark photons have energy, and can be detected? Is not a photon essentially just energy? I struggle mightily to reconcile these concepts. A dark photon is the inverse of energy? Dark Photon + Photon = Nothing?

A dark photon is just a name. It is just the force carrier of a dark force. Since this force has the same properties as electromagnetism, it the carrier is called a dark photon. Like any other particle, it has energy, momentum, etc...

Since the dark photon has no electric charge, it just does not interact with the photon, so that you don't have any reaction like photon + dark photon. This is forbidden. They will be observed through its mixing. The dark photon has a tiny photon component so that it can decay into visible particles like electrons. But since the mixing is tiny, it is a very rare event.

I hope I clarified things :) Otherwise please come back to me!