Anything between 1% and 99% will work - it is up to you!
Sales end when you click the "Stop sale" button. The timing is entirely up to you.
Absolutely! Use the settings on the right of the dashboard to select a collection. Alternatively you can have a sale for your entire shop.
Simple Sale puts the original price in the product's "compare at" field. Each Shopify theme is different so the compare at field may be displayed differently. Most of the time it is present above or below the current price with a slash through it. This lets customers know how much they are saving!