A mana rock that is always on, cant be interacted with, and helps you cast your creatures is enormously powerful, especially when those creatures are generally the most powerful things on the board
With the main set already released, it won't be long until we get the corresponding digital-only cards
Order Ravnica: City of Guilds now
Hitting him for less than his toughness multiple times won't work, as he'll just keep healing
So, winning the die roll still triggers effects that trigger on winning a coin flip, and cards that effect rolling dice have no effect
Remember that they're treated as permanents on the battlefield while in combat, so a graveyard creature attacking will cause anything that triggers when a creature attacks (including its own abilities) to trigger