I would nix the XD subcompact in this case because of the low-profile grip safety that locks the slide from moving when it's not depressed. It works well for the majority of people, but someone with small hands who is already struggling with slide racking will almost certainly have trouble with it.
My strong suggestion to you is that you invest in some quality training for yourself and your wife, even before you invest in a gun specifically for her. With just one day on the range with an experienced instructor, you might find that some of the things she's struggling with really should not be a problem after all. That will open up a lot more possibilities for her firearm.
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