Not necessary, because the argument is about the portrayal of vampires as able to turn into bats. Whether vampires being nocturnal is essential to vampire myths is a separate issue.

Not necessary, because the argument relies on vampire myths from Europe. Whatever is true about vampire myths from other areas has no impact on the reasoning.

Not necessary, because even if they did exist outside Europe, the author is still allowed to rely on the myths from Europe to support his conclusion. If those myths did not portray vampires as able to turn into bats, then the author’s reasoning still works, regardless of the existence of myths in other areas.

Necessary, because if this were not true — if NONE of the European vampire myths that existed before Dracula was published did not portray vampires as able to turn into bats (in other words, all of them portray vampires as able to turn into bats) — then those myths wouldn’t be examples of vampire myths in which the vampire didn’t have the ability to turn into a bat. The author’s citation to those myths would no longer support the claim that the ability to turn into bats is not essential to vampire myths.

Not necessary, because what the writer of Dracula was or was not aware of doesn’t affect the reasoning. The myths that predated Dracula existed regardless of his knowledge, and whatever those myths portrayed about vampires can’t be affected by what the writer of Dracula knew or didn’t know about those myths.