In relativistic terms, "the actual supernova" is an event (an x, y, z, t quad). It's a perfectly well understood entity in relativity. The point in relativity is that different coordinate systems put different numbers on x, y, z, and t, but they all are talking about the same event.
When we see it, we will be able to put a perfectly accurate x, y, z, and t on the supernova in our coordinate system, with t being "8000 years ago". All this has nothing to do with relativity, merely with the finite propagation speed of light.
If we call the moment at which the light reaches us t=0, then we will say that the supernova occurred at t=-8000, because we know where it occurred, and we know what the speed of light is.
General relativity provides a toolset for a complete transformation from "our coordinate system" to any other: a diffeomorphism between coordinates, and laws of physics written in generally covariant form and obeying some constraint equations for values on any three-dimensional submanifold.
If the supernova's matter is that of the standard model of particle physics, you get everything after the comma in the previous paragraph.
So any two observers of the supernova have a straightforward mechanism for relating observations against their idiosyncratic choice (or choices, they aren't restricted to just one!) of system of coordinates. The actual observables are independent of the choice of coordinate systems.
You'll find (mathematical) proofs of this in discussions of the initial value problem in General Relativity; (physical) evidence is the same as that from astrophysical tests of General Relativity.
As a practical matter, realistic observers will only measure a tiny subset of the total observables of the supernova, and different observers will measure a different subset, even if they use precisely the same system of coordinates. Obscuration by dust or distance is not caused by the coordinates in which one expresses distance or the size of dust particles.
> There is no "the actual supernova". That's the whole point of relativity.
No, coarsely, there is a 4d-worldtube along which one finds a WR star, a supernova of type Ib or Ic, and a remnant. Or more finely, there are worldtubes for large numbers of particles that are bound together gravitationally and through standard-model interactions into these more macroscopic objects. In some compact region of spacetime a collection of these particles is best described as the start of a Ib or Ic supernova; their worldtubes diverge (possibly spectacularly) elsewhere in spacetime. Or, if you like, we take a roughly 4-cylindrical section through the spacetime-filling fields of the standard model such that somewhere in that 4-cylinder the excitations of the fields are best described as the start of a Ib or Ic supernova. Really, it doesn't matter, because we know mathematically how to relate each of these descriptions, and have a good idea about why we might choose one description over another when asking various questions. That is the whole point of relativity.
When we see it, we will be able to put a perfectly accurate x, y, z, and t on the supernova in our coordinate system, with t being "8000 years ago". All this has nothing to do with relativity, merely with the finite propagation speed of light.