This has not happened in 1.7.2, so I assume it is fixed. I can not test the newer snapshots as I lost access to Minecraft Realms.
I cannot confirm this. I created a new hardcore world, immediately jumped in some water and drowned, hit delete world -> back to title screen, went to my world list, and the world was not there.
This has not happened in 1.7.2, so I assume it is fixed. I can not test the newer snapshots as I lost access to Minecraft Realms.