HI Rob
"... The problem is the "window" remains the size of the screen and cropping or layering only hides the scrolling text. With the text cropped to a window at the bottom of the screen, the text still has to scroll (invisibly) off the top of the screen to end the sequence. So to have the last line of text display at the end of the musical selection, the scrolling text sequence has to extend beyond the musical selection for the amount of time it takes for the last line to scroll from the visible area, off the top of the screen. ..."
You are correct but note that I mentioned that you could, if that delay is a problem simply apply another text clip of the next (last?) line placed on a higher track (but under the cropped top part of the background static image.).
Even though text 1 is scrolling (invisibly) up to top of the screen behind the top half of the static image, your Text2 line can be positioned so IT scrolls into view as the previous text disappears (or at the correct moment to match the audio.Also the text in my example is not cropped; it is exactly as typed into the text editor. Each line could be full screen width if needed.
If you want to go to extremes then each image of a text line could be placed on tracks 2, 3 , 4 and 5 with the covering cropped background image on track 6. In this way each scrolling text line could be positioned so it starts with the audio.
Worth playing with and (on paper) relatively simple.
(I'll try it and see..)
Nat