Cropping and Blue Screening - 2 unrelated questions.

Question

1) When I straighten a still in Canvas by using the wire frame, to make a house level, say, the cropping lines, naturally, move with it. So a cropping line at an angle will give me a slanting top and bottom line. So how can I crop it so that the picture has a square aspect? Is the only recourse to go back to the original site for the picture, GIMP or iPhoto, perhaps, crop it there and re-import the photo into FCE? Or is there a quicker way?
2) I have added a number of videos above my main video to show, for example, falling snow, and eliminated their black background by Modify - Composite Mode - Screen. Very easy and by no means unusual. So, my question, is, why all the fuss about green and blue screening? Setting it up seems troublesome. A black background for the purpose is much easier to make, and, for that matter, easier to find. I looked down my back yard this morning and this side of the creek are jet black bushes which would seem ideal for such a purpose. This seems such a no-brainer that I think I must be missing something. But it is curious.
Allosaurus

Answer 1

1) I shall immediately turn to the garbage matte section in your book - I'm sure I remember seeing one - and work it out from there. And here I always thought a garbage matte was that rug I bought in Cairo.
2) A black screen. That does give me a thought. When I return to my upcoming masterpiece, " "Orrible Murder at the Castle Doom" I could put one of my granddaughters in a black suit, photograph her against a black background and Voila! a floating head. I wonder if they give out Oscars for original ideas?
Once more. Thank you.
Allosaurus