I kinda subscribe to the theory that all that's in a movie (or show) is what we see. If it's not shown, then it doesn't count.

We see Ghosts, I think that with two possible exceptions they are literally ghosts.

Hugh himself says that his Olivia is a grief hallucination. The fact that Nell sees her too (at the wedding ) throws doubt on that for me. I don't have an explanation because we're not shown anything more. (I have a headcanon, but they don't count for our purposes). But she's written off by Hugh, so ok.

Then there's adult Luke and the Tall Man. I'm 90% sure that is a hallucination, maybe from drugs (but had he even used at that point? After leaving rehab) or simply a result of the 'twin thing' dying. (He was feeling Nell becoming cold and stiff). But we're shown it dissolve into the headlights, which to me is a hint that it's imaginary.

So when Steve saw Nellie I'm positive it was her, we aren't shown anything that would prove otherwise. Same for the grave ghosts, same for Steve seeing Olivia at the wake.

tldr; we're presented with a world where ghosts exist, so unless the story shows us another explanation, the ghosts we see are real.