You can ctrl or shift+select a bunch or all of the missing files, then click relocate, then browse for the first file it asks for and, once you find it, rekordbox will ask if you want to relocate other missing files based on the location of the first one you manually found. Click OK and rekordbox is usually pretty good at finding the rest even if they are not in the same folder. Give it a go.
I use a mac and when I open the missing files manager window there is the usual little red x button at the top left of the window to close the window if you want to and cancel the relocation... I'd guess it'd be the same story for the windows version? (except at top right)