So much white space below webpage

They are right, the more offset values you apply to absolute and relative positioning the bigger the mess gets. Absolute positioning removes elements from the normal page flow and other elements IN the page flow do not recognize them.

Your page also falls apart when the browser viewport width is reduced.

Absolute positioning should only be used for small areas that NEED to be removed from the flow. Not to be used for page layout.

Start over and learn about modern layout methods. For the most part elements in the normal page flow will be normal block elements that will stack naturally on top one another.