I think this is intended, for 2 reaons:
1. simplicity of code. it'd be hard to program it to move around everything
2. abuse. if you made an obsidian cage around where the dragon spawns, it could be abused to 'farm' the final boss of the game. We can't have that, can we?
I think that 6 might be reasonable, but I don't think that it's a bug that you get 4.
do you mean that the bobber is hanging in midair? That's happened to me before, but except visually, there is no other problem.
This is because it allows PvP in peaceful; the player that lights the TNT is allegedly attacking the player damaged
I think that "recreate" doesn't make the same world, it automatically fills in the seed. You can still change the options surrounding the world
the new 1.9 ender dragon boss fight added a lot of changes, the dragon sitting on the portal and spewing 'dragon breath' is one of them