Could I rewrite "and whose grace reminded him of Diana" as "her grace of Diana?"
That is, I hope:
More amazing still, he saw there a girl whose charm reminded him of Mary, and whose grace reminded him of Diana = More amazing still, he saw there a girl whose charm reminded him of Mary, her grace of Diana.
No, your second sentence has thrown off the parallelism and lost the connection...and the sentence structure. Even this-- More amazing still, he saw there a girl whose charm reminded him of Mary, whose grace of Diana-- is overstretching the elliptical ability of the language, in my opinion.