Hhmmmmmmm, excellent question, Nonnie. I had to put on my thinking-cap. Here’s what I came up with!

I think it’d be tremendously rare, tbh? Dwarven ‘magic’ tends to be far more physical and/or mechanical: invisible doors with super-tricky passwords, that sort of thing.

I haven’t ever seen ATLA, sorry!

The stuff we have on portents (which is film-verse only, and not from the books) is also pretty thin on the ground:

Gloin: “Aye, Oin has read the portents, and the portents say: it is time.”
Oin: “Ravens have been seen flying back to the mountain as it was foretold. When the birds of the old return to Erebor, the reign of the beast will end.”

The ravens are mentioned again at the end of AUJ:

Oin:  A raven. The birds are returning to the mountain.
Gandalf: That, my dear Oin, is a thrush.
Thorin: Well, we’ll take it as a sign. A good omen.

Not much to go on!

Using these two bits of info, I guess that Dwarven ‘portents’ are very much based on the actions of the natural world. I also guess that any reading of signs would probably be done by Dwarves who have either been taught familiarity with ravens or other birds, or have spent a LOT of time outdoors, enough to be familiar with the habits and migrations of animals (Oin is very learned and is also an apothecary: he has to collect plants, after all!)

I don’t think that any living Dwarf can see the dead ones except under INCREDIBLY EXCEPTIONAL circumstances – hence, in Sansukh, Gimli only sees Thorin in Galadriel’s mirror or upon the Paths of the Dead.

Mandos rules the Halls of the Dead and has only ever changed his mind once (Beren). The mists between Middle Earth and Aman are impenetrable, and there can be no returning. The Halls of the Dead are even more inaccessible – they’re a world removed within a world removed. None can see the dead, unless they are horribly cursed (the ghouls of the Dwimorberg, the wights of the Dead Marshes, the Nazgul, the Barrow-wights), and thus still residing in Middle-Earth. 

Again, in Sansukh, the only reason Thorin’s Gift works at all is because of a loophole (the

Olórë Mallë), Gimli’s extraordinary perceptiveness, the pool of Gimlin-zaram, and the compassion of a Vala – Aule himself.

When it comes to being comforted, Dwarves would know that after their deaths they are gathered to Mahal’s Halls and cared for there. They have absolute and personal proof of his love for them, after all – their dark names. 

leaving all this aside, though: it has to be said that the idea of a Dwarf medium has a certain Pratchettesque humour about it