2. case: FF body with option to switch to crop mode.......
In point and shoot terms, that's 'digital zoom' and rather useless. You can get the identical effect by cropping in post.
Also, if you set crop mode in camera, in the heat of action, you may overlook setting it to off and lose a lot in subsequent shots.
Hmm, don't know this one, is crop mode shown in the viewfinder, maybe with projected crop lines or only in live view?
If crop doesn't show in the viewfinder, what you see through the viewfinder will be substantially more that what you actually get, that sounds highly undesirable to me.
If crop lines do show in the finder, seems like something else diverting your attention from the primary task. .