As to the "Mannish superstition that Dwarves grow out of stone." I don't know anything of Norse correlations, but you might actually take a look at folklore and fairy tales hailing from Mann itself for the possible source of Tolkiens statement:

http://www.isle-of-man.com/manxnotebook/fulltext/folklore/index.htm

http://archive.org/details/manxfairytales00morr

CS