1. You pay the extra using the card - straight up cash purchase, basically.
2. Depends on the cost of the ticket. With the Aventura card, you basically redeem Aventura points against the cost of the ticket. If the ticket costs more $$ you redeem more points.

To be honest, I feel like Aeroplan is better value for money than the in-house points of the Aventura or Aspire cards. This is especially true since you are flying from a smaller airport which means that you have to connect to a hub - perhaps even twice! Aeroplan just charges a flat rate per area, and does not count the extra stops that you would need to make.

I just took a quick peek at the Aventura site for the flight - and it wants 28,678 Aventura points or $286.78 for the flight on Feb 6. Basically, you can see that it is redeeming for 10,000 points per $100. Aeroplan is better value if you can find a Classic seat for 15,000 points.

Also, beware that switching cards from Aeroplan to Aventura does not mean that you get the bonus - that only applies for new applications. Your points will not convert either anymore.