I am not sure I fully understand what OP (Schlaffie), but let me try asking this:
When I want to select a range of files beyond what is visible, I click on the first file to select. Then, WITHOUT HOLDING DOWN THE MOUSE BUTTON, I scroll down to the last file I want selected. (By this time, the originally selected file has scroll off the top of the window and is no longer visible.) I hold down Shift and then click on the last file I want to select. Multi-Commander selects all the files from the first up to and including the last file.
Does that work for you? I use a Commander-style interface, not an Explorer-style interface; not sure if that makes a difference. Possibly it is not what you are used to, and you are looking to do it your way specifically rather than there being a possible way to do it. But definitely you can mark more files with the mouse than are visible in the window.
Hope that helps!