Oh I see.. It makes sense now. I remember watching "Le Nozze di Figaro" and didn't really know the reasons behind making a women playing the young man character.
I'm quite novice when it comes to opera but I remember watching that one and was surprised that I was halfway into it without blinking.
If I remember correctly the women played a young man and then during the story she plays a women as a "disguise" then later reappeared as a man.
I'll check out "Fidelio"
Yes, The Marriage of Figaro gets pretty complicated at times. The confusion is a comedic vehicle. I'm glad my story helped you sort it out a bit. You might want to see The Barber of Seville, which is basically the first part of the story that is continued in Marriage of Figaro.