Making damaged items not smeltable is just for balance, same as not getting all the materials back, that would eliminate the possibility of infinitely smelting the same items over and over for XP. I'm not against mob farms, I think they are a clever way to play the game, and many people enjoy building them and improving them. But getting such a low return rate on smelting items is a bit much, it should be higher than that, but not 100% either, half sounds about right. At one nugget per item, it's not worth doing at all, so why they bothered to put it in is strange if they are going to make it so useless.
The way mobs spawn and despawn I have no issue with, same as going on the roof of the nether, if a player is willing to spend the resources to do that then I have no problem with it. Bedrock floors and ceilings are just there to cover up a limitation in the game engine of not being able to generate an infinite height landscape, like invisible walls in FPS games to keep you from going outside the map, it's a crutch to prop up a limited game engine and if players can find a way around it and make it useful, then more power to them, another creative and interesting way to play the game, IMO.
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