How to save a Dendrobium 'Samurai' (Sameri) - partly dying and new growth

[HiOrcDen]

I have a pretty large Dendrobium Sameri (Samurai). It has been gradually, steadily losing leaves since soon after purchase.

Before the last of its previous leaves died off, it put out a new growth. This growth has sprouted steadily and seems healthy, though there are some dark marks. I have used a 3 in 1 treatment.

I hope the pictures helps clarify. As you can see most of the plant is barren, except for a single green growing stem.

Also, there is a spot which I am not sure is keiki or air roots (if keikis can form naturally, that is) Should anything be done with air roots? And if it's a keiki, how do I cut to propagate?

The growing medium has collapsed somewhat. Should I entirely repot the plant, including roots associated with the new, healthy growth? (That is, if certain roots can even be associated with certain growths) I am wondering, because might repotting interfere with the conditions allowing the healthy growth? I am leaning towards full repotting... as perhaps even the new growth will soon lose its root support.