The achievements you see ingame in the inventory follow a rough path of 'tiered' achievements that go up as you gain them; once you get the final one (which is beating Moon Lord) they will dissapear, as there are no more 'tiered' achievements to get. This list does not include every achievement in the game, only ones tied to progression for the most part. This unlocking happens with terraria as a whole instead of per world, so once you have beaten Moon Lord at least once on any world, (and got every achievement needed up to him) then it will not appear again from then on.
I don't think your second problem is related to this, as the credits do not do anything that could cause host n' play to not work. Any way you can give more information on that? Have you tried restarting to see if it works then? Did it work before this?
