New answers tagged layers
I can't believe no one suggested this yet: Just use the rectangular marquee to select what you want to crop down to, and COPY it to your clipboard. Then delete the entire layer and PASTE what you copied to a new layer. This is especially useful if the layer you're cropping is larger than the canvas, in which case the select-inverse technique is messy.
As the previous answer demonstrates, you can merge the different exposures using layers. But it's a lot of work. I'd suggest trying enfuse, which I've been quite happy with. It produces realistic looking output, not the dreaded "HDR look", and does so with pretty much no work on your part. It runs via the command line. However, if you have Lightroom ...
I think using Mike's suggestion of creating masks using the difference modes might be the only way to do it. What you want is the content of one layer that is not is another, which is what the difference mode highlights.
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