I want to know what studies have been done on the European version of
the swan maiden folktale since 1950. I am particularly interested in
archetypal analyses of the tale.

Swannie, have you already read Emma Jung's essay, "Anima as Elemental
Being", which includes some discussion of swan maidens?

There's a bit about it on this webpage:

"Mrs. Jung surveys myths and folktales which portray alluring females
who either are in large part animals--swans, snakes--can transform
into such creatures . . "
http://www.jungatlanta.com/anima.html

Good luck - Leli