Yes, Personal Discount is free, up to 50 coupons generated per month.If you generate a coupon for your new newsletter subscribers, it means you can offer a discount to 50 of them per month.
No, it's a soft limit. The application will continue to work. We will notify you and give you some time to upgrade.
Due to some limitation in the Magento platform, you will be not able to see the expiration date of your coupons in your shop interface. You can however see the expiration dates in your Personal Discount dashboard. Personal Discount takes care of removing the expired coupons.