Instead of replacing Madrid's footline entirely (which didn't work), use addtobeamertemplate to prepend logo and setbeamercolor to make title text white. Both via BEAMER_HEADER (after usetheme).
Instead of replacing Madrid's footline entirely (which didn't work), use addtobeamertemplate to prepend logo and setbeamercolor to make title text white. Both via BEAMER_HEADER (after usetheme).