If there is more than one Propaganda on the battlefield, the cost is cumulative.

If there are multiple attacks in a turn, then you have to pay for each attack.

Paying this cost is not an instant or any other kind of ability, it is an additional cost on the declaration of the attacker.

The payment is made during the declare attackers step at the same time you are declaring the attacker.

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

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