China dishes

I saw you wrote:
Glass, plastic, glazed china, glazed dishes, Corelle, Pyrex, Corningware, or glazed earthenware do not need any Koshering, since they do not retain any flavor.

Does this mean I can use my year round china set (meat) for pesach (meat) as well without any koshering?

Correct

Does it make a difference if it was used for cold or hot things?

Yes for ashkenazim if it was used to cook chometz, you can’t use on Pesach. These are not normally cooked with

What if one didn’t cook on it but merely at hot chametz on the plate, can one use these things on Pesach?

Yes