lens - What are the differences among these Canon telephoto zoom lenses?

I am using Canon 500D with the kit lens (18-55). I think I have done enough with 18-55 and want to try out next lens. I have already decided to purchase 50mm prime. I also wanted to get a Zoom lens. When I looked around Amazon, I see many varieties of zoom lenses.

I have 4 questions.

First lens in the above list is $524 and rest everything under $200. I am wondering why such a huge difference? Is it because first one has USM in it?

I am obviously confused and request for expert help to choose which one to buy. Is there a problem in buying lens without USM? Will the low pricy one reduces image quality?

My friend uses Nikon and is there any zoom lenses available which can be mounted both on Canon and Nikon?

I have also seen people recommending Tamron AF 17-50mm F/2.8 as a choice for Zoom lenses. Is this the better choice? If yes, how it is better from longer ranges like 70-300? Since this lens offers F/2.8, can this be also used for portraits?

I shoot whatever I like, but mostly landscapes. Any help would be appreciated!