No, mobs will not spawn in water or ontop of half slabs.
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The only mob that can spawn in water is currently a squid.
In Minecraft Classic, mobs can't spawn, so they can't exist naturally. If you play the Survival Test, and when mobs spawn there, you can save it and open it in creative mode so then the mobs can stay there. However, in Minecraft Creative Mode, mobs can spawn, but they can't hurt you because you have no healthbar.
No, the AI for mobs in minecraft has been improved so mobs don't spawn in strange places. You can manually spawn them in lava with that mob's egg.
Yes, in the Nether Zombie Pigmen spawn on the netherrack but I don't think they spawn on Netherrack in the Overworld.
Yes, but the Pigmen and Ghasts won't spawn.