no
*if* they get over 50% or the network they have a "chance" of taking over the network (if they wanted to), but even at 90% of the network there is still a 10% a chance they would not succeed.
false, at 90% of the network if they hide the block chain they have 90% chance of generating one block, but the chance of generating two blocks before the main chain generates two blocks is GREATER than 90%