You're right about the similar melting points of silicates and iron, I hadn't considered that, but the events I proposed are rather temporary, in astronomical terms. It's still possible for some remnant of a planet to survive, although you're correct in pointing out it may not necessarily be the core, it may include part of the mantle, or just the inner part of the core.
Then again, it's possible for the whole planet to erode away, too.

Cores are pretty hot already, they may even be partially liquid, and one rapidly exposed in this fashion may react quite strangely to the sudden removal of overlying layers. Still, I find the idea of a polished sphere of nickel-iron orbitng a dim white dwarf, like some kind of giant ball bearing, a fascinating conjecture. It might have some weird magnetic properties, too.