OP, I'd suggest you x-post to /r/AskLiteraryStudies. I'm not an expert in this topic but AFAIK a handwritten manuscript for this book exists; they may have more information about any debates re authorship.

There is some speculation that Frankenstein may have been a collaboration with Percy Shelly. I have never heard anyone claim they outright did not write it.

We could compare the 1818 version to the 1831 version. One written when Percy Shelly was still alive the other released long after their death but I really not sure that could ever prove anything. Having read both versions the differences are fairly minor. You could claim if the work was already written by Percy any further editing by Mary would be fairly straight forward. You could claim this was Mary producing their definitive vision of the story and removing inputs Percy may have had to the original story. Such claims could only ever be theories though.

Mary Shelley did go on to write other books after Percy Shelley had died so they certainly seemed to have had an ability to create works that seemingly Percy would have had no input into. Obviously none of their other works have the same level of fame but in their own time they did have some level of personal success outside of Frankenstein.

I am actually reading The Last Man at the moment and they were clearly a talented writer and it's also a fairly imaginative book for it's time so this seems to be fairly consistent with the idea they could come up with stories that were fairly unique at the time such as Frankenstein. This again proves nothing though.