BMP uses RLE (run-time length encoding), which can actually be very efficient for images that use relatively few colors.
Hench the mostly bit. Also BMP does not use Run Length Encoding by default. It can use (as an option) RLE to reduce the size of very large, palletized images. For any 24-bit image that contains no large all black or all white sequential areas however RLE is not going to do much to reduce your filesize. but the less actual colors you use, the better the compression gets.
Only if you're using a paletteized image. BMP filesize is not a good indication on how much actual memory the decompressed image would take up.
No. But it's better, and easier to understand than "nothing".