You can do macro with the 50, what you give up is working distance. You have to be closer. That's why the 100 is preferred for tank shots. If you're taking a picture of a bug you can just move closer. That's hard to do with glass and water in the way.
Next question. USA lenses are designed for the US market and carry full manufacturers warranties. The "imported lenses" are not made for the US so you can't send them to Canon for warranty repair. Those are often referred to as "grey market" lenses. Depending on the seller, they might include a 3rd party warranty but I'm not sure that I'd want to rely on that. Example: I just got my 24-70 back from Canon for an out of warranty repair. Anything that you have to send them will cost at least $150. That's often more than the difference in price between US and imported.