Probably depends on the spell? Things like the Disarming Charm would definitely still work on a weapon. For other dueling charms, maybe they can be blocked with a thicker weapon since that’s a solid object

The diary was very significant to Voldemort, who only chose horcruxes that were valuable to him in some way. However, once an object has been turned into a horcrux, it’s just as durable as any other horcrux and can only be destroyed by certain means.

This feels like an impossible situation- everybody has/values some trait corresponding to a house, even if it isn’t obvious. However, if we assume that this could happen, I’d be inclined to say Hufflepuff, as the founder would historically take anyone

“To different people”, as in, to different people drinking the same potion? Polyjuice represents the essence of a person (whether good or bad) and probably wouldn’t change based on somebody’s perspective of that person