Muggles' Guide to Harry Potter/Magic/Sectumsempra

Overview
Sectumsempra is a very dark curse that causes a cut (Latin "sectum" = "cut") which will not heal ever (Latin "semper" = "always").

Extended Description
It is possible that Severus Snape was planning to use this jinx on James Potter when he was being picked on in the scene that Harry witnessed in the Pensieve, in Harry Potter and the Order of the Phoenix. However, as Snape never got even one syllable of the jinx spoken, we cannot be sure.

In Harry Potter and the Half-Blood Prince, Harry finds this curse in the Advanced Potion-Making book that was previously owned by the Half-Blood Prince. There is no description of its effects, the only notation is "For enemies". Out of desperation, Harry used it on Malfoy, as Malfoy was preparing to hit Harry with the Cruciatus curse. It apparently causes the caster's wand to act as a very long knife, thus resulting in the target getting huge cuts across their body. The way the caster moves their wand when saying the incantation decides how the cuts are made. Since Harry's hand was shaking badly as he pointed it at Malfoy, Draco got large cuts across his chest and face. Professor Snape, summoned to this duel by Moaning Myrtle's screams, manages to close the cuts, and then takes Malfoy to the Hospital Wing, saying that he will need essence of Dittany to heal them up all the way. This evidently works, because when we see Malfoy again, there is no mention of scarring.

Snape used this spell on George Weasley in Harry Potter and the Deathly Hallows, cursing his ear off. Mrs. Weasley remarks that as his ear was severed by Dark magic, it will be impossible to grow it back. Mentioning that it was Snape who did it, Lupin remarks that "Sectumsempra was always a speciality of Snape's."

Analysis
The spell is said to create never-healing cuts, yet Snape was able to partially heal Draco with an unnamed spell. It is reasonable to suppose that since Snape either invented, or did a significant amount of research into, the Sectumsempra jinx, he more than likely came up with, or discovered, a counter spell for it.

Questions

 * 1) Given that Snape was revealed to be the Half-Blood Prince, was it Snape that created this spell?

Greater Picture
Much later in the book, it will be revealed that Snape had been attempting to use this jinx on the Death Eater then targeting George, to remove or at least disable his wand arm; his hitting George and removing his ear is accidental.

We note that Snape claimed to have created this spell, along with others, in the duel with Harry at the end of Harry Potter and the Half-Blood Prince. They were certainly written in the textbook he had owned, and there seems to be little knowledge of them elsewhere, though Lupin, discussing the Levicorpus charm with Harry, seems to suggest it was merely a charm that had gone in and out of style.

Answers
Given that Snape was revealed to be the Half-Blood Prince, was it Snape that created this spell? ––    Yes