Who is the magical girl who becomes Walpurgisnacht? Since witches are actually magical girls who, when their soul gems are fully tainted, become witches, that means that Walpurgisnacht was a magical girl. Since Madoka in two realities became a witch that could destroy the world/universe, that would mean there was a magical girl who was, at the time, much stronger than Madoka (before Homura's trips to the past compiling Madoka's fate and power).

Which magical girl became Walpurgisnacht? Were they like Madoka, and had their fate compiled from overlapping timelines?
There is no information about who it was that became Walpurgisnacht. All we really know is that Walpurgisnacht is, in some sense, a fusion of multiple witches (or magical girls, I guess).
Official description from July 2011 Megami interview with Urobuchi:

In the last episode we saw the &quot;Walpurgisnacht&quot;, but what kind of witch is it specifically?
It has the destructive power to bring about natural disasters powerful enough to blow away an entire town, but originally it was a single witch. It's a witch that has grown from the combination of countless other witches. Walpurgisnacht combines with other witches in the same way two powerful tornadoes are able to combine and become larger. It's essentially a &quot;conglomeration&quot;-type witch. Because it's so powerful, it rarely shows itself.

See the Madoka wiki for more details.