In retrospect, having Mina's and Jonathan's son born on the anniversary of Quincey Morris' death in Dracula serves an additional purpose besides just being a sweet coincidence. It means that their son was born, at the very earliest, 12 months after the team destroyed Dracula — further meaning that, contrary to the ridiculous Epileptic Trees planted in Fan Sequels, there is absolutely no way Dracula is the father of Mina's son.
Regarding the awkward representation of Funetik Aksents: The story is told as a series of diaries kept by late 19th century toffs and aristocrats, who would naturally have some difficulty transcribing the working classes by themselves. Still doesn't excuse Van Helsing when he repeatedly falls into German (instead of Dutch, which supposedly is his mother tongue).
Also regarding the Funetik Aksents: Dr. Seward's diary was kept in phonograph, meaning that while he recorded Van Helsing's dialogue, he was mimicking the Professor's accent. Additionally, Van Helsing's lapses into German may be the result of the English characters not knowing enough of either Dutch or German to transcribe the former language properly.
So you need an elaborate ritual with prayers, wooden stakes, and garlic to take out lesser vampires, but Dracula himself you can just stab with a normal knife? Even after he shrugged off a shovel to the head? And you know he's dead because he turned into a cloud of dust, even though this is an established part of his powers which he used to attack Lucy?
It was never said that the ritual, prayers, stakes, or garlic were strictly necessary.
There was no elaborate magical ritual when Lucy was staked, just a stake and a hammer. That's only one way to kill... er, destroy(?) a vampire, however; check your local lore to see how sharp, steel or Cold Iron objects like knives or needles work just as well. Van helsing also mentioned silver bullets and branches of wild rose as possibilities. You know he's dead because of No Ontological Inertia — his victim (Mina) is no longer a pending vampire.
Much is made of vampires having little-to-no blood flow, which impedes the progress of van Helsing's serum in Dracula's body. But then, shouldn't Dracula not be able to have an erection when he's having sex with Lady Jayne?