I know this discussion was a long time ago (2001), but I just found the question on whether Mary Shelley actually wrote Frankenstein and felt compelled to add a few details to your otherwise fine response.
To start with, Shelley wasn’t really a Victorian writer–at least, Frankenstein was first published in 1818, well before Victoria was crowned.
Second, the novel was published anonymously, as most novels were at the time, so the idea that PB stuck his wife’s name on something he considered beneath him goes out the window on that score alone. However, people suspected this was something that PB might have cooked up, and rumours did spring up shortly after it was published that he had written it–he did, in fact, write the Preface to the original 1818 edition, and there is some evidence that he offered some editorial advice to Mary.
Finally, your account of the story contest is a little misleading–as Shelley herself says in the Preface to the 1831 edition–the version that most people now read–Shelley, Percy, Lord Byron, and Polidori had rented a house in Switzerland for the summer, but the summer of 1816 was especially wet and cold. Byron proposed the story contest, as a way of passing the time. It wasn’t really a gathering of the literary elite–Byron and Percy Shelley were on-again off-again friends, and Polidori is now known mostly as a hanger-on of the Shelley-Byron group. The “contest”–which no one except Mary seemed to take too seriously–didn’t have any prize attached to it–it really was something devised to pass the time. Byron and Percy never finished their stories, though bits and pieces of their ideas turned up in other works. Mary took about a week to come up with her story, and was inspired by a conversation among the guys, who were talking about electrical experiments and whether a dead corpse could be reanimated. That night, so she tells us, she had a dream in which she saw Frankenstein standing over the corpse he has reanimated, horrified by its appearance, and running away in terror, hoping the thing will die of its own accord. Then, he sees the yellow-eyed creature standing over his bed as he sleeps. Next morning at breakfast, she announces to the boys that she has “thought of a story,” and the rest is history.
Perhaps the best evidence that Mary Shelley wrote the novel is that she revised it in 1823, a year after Percy’s death, and revised it even more in 1831, when she was asked to include her novel as part of a reissue series. If there had been any question about her authorship, the publisher would not have accepted a heavily revised version of the work. Besides, Mary Shelley was absolutely devoted to preserving every scrap of Percy’s works, issuing a fair number of his unpublished poems after his death. There is little likelihood she would ever have claimed something as well-received as Frankenstein after Percy’s death if he had actually written it.
One final note–do read The Last Man, her second novel. It’s even stranger in some ways than Frankenstein, and is oddly even more relevant to today. It deserves a bigger reputation than it now has.