The mixte would be great for making a nice upright bike for light riding. You can't beat a mixte for elegance. However the schwinn woulds like the perfect size. My girlfriend is 5'5" and rides 48-50cm frames because the TTs are too long on anything larger... a 50cm TT is ideal for her with a drop bar, with some uprights though that changes. Women tend to be longer in the torso and for this reason it's difficult to set up a vintage bike well for them. Thankfully women specific road bikes are available today... But I digress. the 21" mixte should be around 53-54cm which may be an alright fit with the seat quite low, then again my girlfriendf who is the same height as your wife would not be able to ride such a large frame.... she would have NO seatpost showing which isn't good. You'll have to throw some wheels on there and find out... remember changing wheels size may alloy her to put her feet on the ground more easily buy it will not change how the bike fits.
1 Super Record bike, 1 Nuovo Record bike, 1 Pista, 1 Road, 1 Cyclocross/Allrounder, 1 MTB, 1 Touring, 1 Fixed gear