All good points, but at the same time, MP has much smarter opponent(s) in terms of conspiring against you. In a game with AI's and just one other human opponent, that human opponent can also use various game mechanics to know you have that many cities, and, wa-la, that human opponent bribes another AI (or several, if given enough time) into war against you and doesn't need to build an impossibly large army themselves.
And in a game with only human opponents, this goes doubly.
The only situation that really applies to what you're saying is 1v1 MP with no AI's. In that case, you're right, it's a straight-up balanced game where Spain getting 2 early cities would be a huge boost that would be hard to take away. In that case, 1v1 MP probably is easier as Spain than single-player Deity or Immortal. But in all other cases (MP with AI's, MP with more than 2 players), I would imagine that MP would be harder because human opponents are more likely to conspire against you or to attack you simultaneously by coincidence even, much more likely than AI's that you can basically cheat for gold through shoddy diplomacy.