Could you tell me how the female duck was confirmed to be an entirely different character if she was not really Daisy under a different name?
Well, first of all Carl Barks, who wrote the plot of "Mr. Duck Steps Out", said in an interview that
"
Donna Duck was invented and incorporated into the story Don Donald before I came to the studio. I never associated that fiery Latin senorita with the more American-type Daisy Duck that came along later." (source: goo.gl/eQLLyN)
Plus, Daisy and Donna appeared together in many comics. Here is an example:

Finally, there are books written by Disney scholars, and approved by Disney, in which Daisy and Donna are presented as different characters:
https://lists.nafsk.se/p...2002-October/014416.html Even websites of various Disney branches give "Mr. Duck Steps Out" as Daisy's debut:
http://www.topolino.it/archivi/paperina/ As for the misspelling of Dewey, well, I don't think I need a source for that.