Overflowing content can cause a double scrollbar and in your case, it's the bottom separator of your last section. Please disable the bottom separator.
As for the fixed footer, please remove all the codes posted in this thread. A simple way to push the footer down is to add a tall Gap element for pages with little content in order to simulate long content and avoiding conflicts like you've experienced.
If you want a sticky footer, you can use the Header Builder's Bottom Header. For more details about the Header and Footer Builder usage, please see https://theme.co/apex/forum/t/pro-header-and-footer-builder-introduction/103
I'll show you a better custom CSS solution that is simple and avoids conflict with other selectors but please note that this is only a guide. Issues that will arise from the use of this nor further enhancements will not be covered in our product support. You still need to remove the codes provided previously.
Here's the code:
Adjust 50 depending on the height of your header and footer.
Hope that helps and thank you for understanding.