All files should be 24-bit WAV files for every single instrument and vocal track. This ensures a higher quality mix for many technical reasons.

This is to ensure everything lines up exact. Whether the first sound of a track is silence or something else, it ensures things do not become misaligned. Otherwise, your performances may not sound exactly as desired.

Yes, this is preferred. To provide you the best possible mix. Otherwise your vocals will never sound as in-the-mix as they could.