Welcome.
1. I can't tell you why WMC works with incorrect data. As you've discovered, MC doesn't. Be assured it will work fine when the data is corrected. You could confirm that by fixing a few manually. To the extent your files are organized (i.e., in the file system) by Genre, Artist and Album, you will be able to extract that data from any number of files at once using Library Tools > Fill properties from filename. If that's not a compete solution (e.g., Genre is not part of the Filename), you'll find it easier to fix the rest manually by sorting the file list by Filename and 'filling in the blanks' (e.g., a missing Genre will be the same as that in the following track).
2. Images have [Album] set equal to the date on import. I suppose the idea is that might be useful until they're tagged differently. In this case too, you can use Fill properties from filename to extract whatever data is included in the existing filepath. How you deal with this on an ongoing basis depends on how you decide to use MC to organize photos. If there's no other software involved in the process, you might import the images directly from the camera, tag them as you wish, and then use Library Tools > Rename, move and copy files to set new file pathnames based on those values. If that task is to remain with other software, you can use Auto-Import rules to extract data from the pathnames on import (i.e., doing the Fill properties from filename function automatically).