They are in fact a different species. They share a common genus though which is enough to breed. In this example of bears, the light brown bear represents the human, the black bear represents elves, and the panda represents Drow. So to clarify, Drow aren't in the same Genus and are more distant to humans and elves, but due to many things, they're closer to elves than humans so do have a chance at breeding. But it's very hard for a panda (drow) to have a child with a light brown bear (human)The answers are possibly and no in that order.
To elaborate, the fact humans and elves can interbreed means that in terms of biology they aren't actually different species. An apt comparison would be to think of them more like different breeds of cats or dogs. Big differences in both phenotype and every other trait but still the same species.
Although personally I prefer to draw a parallel to the situation of early hominids. There was in fact a time in our past when homo sapiens lived along side and interbred with other types of humans such as the Neanderthal. But eventually we just outbred them and became the dominant and than only species of human left. Even though Europeans in particular still retain some portion of Neanderthal DNA much like the various people in this setting with traces of elven ancestry.
What seems to have happened in this world is that through geographic isolation a relative to humanity managed to avoid this faith much like the various odd creatures we find in the heavily isolated places of our world, for example Australia. And much like those this relative seems to have evolved some very odd and distinct traits. Even if they don't have venomous claws, a duck bill, beaver tail and sweat milk all at the same time. But they seem to have remained mostly compatible.
And we know that in some but not all cases related breeds can share blood between them. So it's definitively possible that human to elf or elf to human blood transfusions might work. But it's also possible that they wouldn't. It depends on just how similar / different their blood groups are. But it's, as said not out of the question.
As to if you could fool a DNA test using blood transfusions that is very unlikely. Any amount of transfused blood would diffuse into your bloodstream fairly quickly. So in order for it to be even remotely plausible that the tester would pick out the injected blood and it alone for his sample you would basically have to do something very extreme like replace most of the bodies blood with that of the other species. And whilst that sort of procedure is definitively possible and has been done it is something that would have to happen under controlled medical conditions in a proper medical facility and thus leave a written record.

Therefor it's not great to compare elves and humans to different dog breeds. But it is fair to compare elf breeds to dog breeds. Elves share the same body plan but with different sizes, some have better eye sight, ears, etc. Minor but noticeable changes, much different than humans are to each other.
Neanderthals aren't humans at all, nor are they a different type of human. They are a different species in the same genus just like elves.
Humans are Homo-Sapiens
Neanderthals are Homo-Neanderthalensis
So elves would be Homo-Sylian

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