25mm x 1.6x = 40mm
No. It's EF-S so it's 24mm on APS-C. If you put EF 24mm on APS-C, then you will get equivalent around 38mm.
Uh. I'm almost sure that an ef 24 has the same FOV as an ef-s 24. The added "S" just means that it's only mountable to crop bodies.
Depends on what the image circle produced by the lens really is