Electoral College just prevents a few large cities from controlling the entire country.

It's the winner take all aspect of the presidential vote which means it is most efficient to have two parties.

A third party similar to Dems means the GOP wins; a third party similar to the GOP means the Dems win...either of which results in minority rule.

So, third parties harm themselves in such a system, because they end up empowering the party they are most different from. You gave examples of that above.

In parliamentary systems, the third, fourth and fifth parties can combine to form coalition governments...so they can still affect policy when they aren't first. Not so with FPTP or winner take all voting systems like ours.