Although the token created is an attacking creature, it was never declared as an attacking creature. This means that abilities that trigger whenever a creature attacks won't trigger when it enters the battlefield attacking.

While resolving Carefree Swinemaster's triggered ability, you can't pay the cost multiple times to create more than one token.

You choose which player or planeswalker is being attacked by the Boar token as it enters the battlefield. It doesn't have to be the same player or planeswalker that Carefree Swinemaster is attacking.