**SPOILERS HBP and DH**
I think the big battle you are talking about is the one in Deathly Hallows, and almost half of the characters die in that. The only significant person who dies in HP6 is dumbldore. I could be wrong, since i am re reading HP6 and havent gotton that far yet in the story.
Memories that were missing are Ogden's memory of visiting Marvolo, Morfin, and Merope Gaunt. Merope was voldemorts mom, marvolo his grandfather, and morfin his uncle.
The memory of Morfin in which tom goes back to the shack and discovers his father was a muggle that abandoned his witch mother. Tom murders his muggle father and grandparents and makes Morfin take the blame (by implanting a false memory)
The memory of Hokey the house elf, in which Tom Riddle visits Hepzibah Smith and she shows him Helga Hufflepuffs cup and Salazar Syltherin's locket (which he knew was his mothers). He apparently murders her and steals those things and makes Hokey take the blame (false memory again)
In the deathly hallows, it is discovered that Dolores Umbridge has the real locket horcrux. R.A.B. had it first though.
Snape kills dumbledore so the elder wand's line can be broken. To become master of the wand, the person must kill the owner. However, Dumbldore's death was planned out, so Snape didnt really murder him. It was all done purposefully and all planned out. Snape wasnt the real owner. The real owner of the elder wand is Harry, howver I forget how that happened. So in Deathly Hallows when voldemort kills snape, he didnt possess the elder wand. Oh, and also he made the unbreakable vow to protect Draco and to make him appear on voldemort's side still.
Overall differences is that: Dumbldore picked Harry up from the Dursleys. Tonks found harry on the train. Ginny and Harry first kiss after they won the quidditch cup, in front of everyone celebrating. Tonks and Lupin dont get together until Dumbledore's funeral. Harry asks dobby and kreacher to spy on malfoy, but that never happens. Slughorn actually asked Professor Sprout to help him, whereas in the movie he was sneaking into the greenhouse.
There coukld be more, ill edit if there is. Sorry if i ruined Deathly hallows, but some of the questions you asked have the answers in that book.
K, hope this helps xD