Hi there, @Leocatal ...
I can help with the first part of your question...
When you are on your "Grades" page and scroll all the way to the right, it sounds like you are seeing some additional columns to the left of your "Total" column. They may have the same name called "Assignments"? If you have columns like this, each of these are the result of having Assignment Groups on your "Assignments" index page.
So, if you have two "Assignments" columns on your "Grades" page, when you look at your "Assignments" index page, you should see two Assignment Groups each called "Assignments". On your "Assignments" index page, do you have assignments in each of these? If one of those doesn't have any assignments in it, it's probably ok to get rid of it. But, these are also where the weighting is set up. So, it sounds like one of those "Assignments" Assignment Groups is weighted at 40% while the other is weighted at 0%.
I hope this helps in some way!