Not equal, but similar enough. You can double spend for a while until no one trusts the currency or low confirmation numbers anymore.
Not at all similar. Outpacing the chain in POW is super linear in the number of blocks, that means the more blocks you need to produce, the higher the cost to you and this relationship is a curve which curves upwards towards 'very high cost'.
Once you own some masternodes, the cost is zero for the attacker.