Unless some effect explicitly says otherwise, a creature that can't attack you can still attack a planeswalker you control.

This has the supertype world. When a world permanent enters, any world permanents that were already on the battlefield are put into their owners' graveyards. This is a state-based action called the "world rule." The new world permanent stays on the battlefield. If two world permanents enter at the same time, they're both put into their owners' graveyards.

In the Two-Headed Giant format, you still only have to pay once per creature.