1. As much as I am aware, Kushina didn't change her name after the wedding, kinda-sorta very opened marriage. Probably Hiruzen just left Naruto with his mother's name (just a theory) to protect baby Naruto from people, looking for the son of Namikaze, the Yellow Flash. But it's just a plot hole.

2. Despite the sacrifice of Minato, people were afraid of Naruto, believing that the chaotic evil chakra of the Nine-tails will simply devour his own and turn him into evil entity. Pretty much they believed the boy is pretty much dead and it's just a shell for the 9tails. This is (possibly) symbolic of how people are blinded by hatred.

3. I believe that people did not recognize the hokages, mostly because very small part of them remember them alive and I hardly believe that the faces on the hills above Konoha are really that accurate representation. Also most of the shinobi on the field are not from Konoha and dont recognize other's villages kages. (DESPITE ON THE BACK OF MINATO LITERALLY WAS SPELLED FORTH HOKAGE!!!!!!!!!)

4.Karin probably either had affinity toward that skill, or was taught to it before meeting Orochimaru, or most likely - PLOT DEVICE and LAZY WRITING.

5. Plot device and lazy writing

6. Same from no one

7b. I believe the technique Orochimaru used was still not refined nor perfected. Kabuto completed it to the form we see in Shuppuuden.

8. I believe Sasuke was either able to increase his chakra pool with the help and training of Orochimaru. Or he was able to train his body to develop better chakra control, allowing him to use less chakra with better results. Or lazy writing.

Pretty much lazy writing.