To be honest, I agree with Lark. Many deities in different cultures sometimes have the same myths. Or in other words, same tributes.
Like Demeter and the Persephone myth, it has some attributes that are alike with the Isis myth.
However coming from a christian backround, i don't believe all these deities are linked. It seems a little far fetched...that's my veiw.
We all have different religions, and different gods, veiws, ect. I believe that no these deities are not the same one.