Hub knuckle yes as you describe it holds everything including the track for the wheelbearings If thats worn you will find that the wheelbearing track will fit in far too easy. They should be a very tight fit and need a large socket and a hammer to fit them in correctly. If the track is slotting in or even spinning round in the metal knuckle then its worn.

Also the track that fits on the hub itself should be a tight fit and no play at all!

TBH i cant see that it would be that causing your discs to warp, but it could cause virbration under braking causing a feeling like there warped.

I'll try and pop in there and see if they got anymore Hubs instock