Sorry I don't think you can save your pine tree.

I dont either - but try watering it with plenty of light?

Not sure about the white mould - I've repotted conifers before and never seen that. Most likely explanation for the death of your conifer is lack of water after repotting, or rather, insufficient water supply, particularly as its roots were disturbed during the summer rather than in winter.

I'm guessing without seeing it that the white mould is one of the mycorrhizal species that is actually very good for the plant. It is one of several potential species of fungus that associates with the roots of the plant helping it obtain nutrients from the often nutrient poor acid soils that it often grows in. "Removing" this will have not only been ineffective but also will have damaged the roots I would have thought!

All the Pines we sell at work have this fluffy white fungus (mycelium) on their roots.

The only other thing I can think of (which is quite different) would be root aphids, specifically woolly aphid which produces a fluffy greyish-white waxy excrescence. I have also seen these on many conifers, especially Pinus. These are completely detrimental to the plant of course as they suck the sap and reduce the vigour and health of the tree sometimes even introducing virus in rare cases.