I often use Snapseed and I don't really think it can so the defringing fix. I do not know what a keystone distortion is so I do not know how that would be fixed. I agree with the other poster using the many filters and textures would surely help fix problems in an image. Give it a few tries. I say few tries because Snapseed may bomb out on one image and do a splendid job on others. It is hot and miss when playing with images.
I like Snapseed and I really don't have negatives at this time to rant about. But I can say that every image is different and what worked for one may not work for the next so keep playing!
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