Hi,
1. Regarding your checkout process, when you click on your mini-cart-module you're redirected to the first step of your checkout workflow, so I think that you have just put the cart option in the first step.
2. Here is the purpose of this option through the documentation :
Force a menu on the checkout : You can set here a specific menu where your users will be redirected when displaying the checkout page. That allows you to display different modules/templates on the checkout than on the rest of the shop. If you don't set it, the user will keep all the modules and templates of the rest of the store when displaying the checkout page.