1) The way I understand it is it's just from the first issue, the character as brought to DC and they would share the trademark. It would be sorta like them getting the rights to the Earth-2 Superman although it would be minus flying, super-senses, Jimmy Olsen, Daily Planet, kryptonite, etc. The costume would be distinctively different such as the S-Shield and his boots. What I'm unsure is if this means DC would owe some kind of royalties to the families every Superman comic they publish as every usage of the character would be based on that first issue.
2) The reason for the name change, if the GA character took off and warranted his own book, it couldn't be titled Daredevil as Marvel has that TM locked up. By changing the name, it allows him to be featured in his own books with his new name. Image/Larsen chose not to change the name which is within his right. But, it means he cannot use the name as part of a title or selling underoos or what have you.
3, 4, 5) DC owns the trademarks ie the rights titling books by those names, marketing the characters' images and names on lunchboxes, etc. But, they don't own the copyrights which means anyone can reprint the original stories and create derivative works as long as it doesn't violate their trademarks. So, conceivably, I could publish a book called BIG THREE (old Fox title), put the GA Blue Beetle on the cover as long as I don't bother to name him on the cover AND have individual stories inside of each of the 3 Blue Beetles prior to DC's take.