If you have another lens installed just go ahead and ignore the wide FOV image warnings. There is no need to update the exif to reflect the new focal length.
The exif GPS location is used in the georeferenced workflow, clearly. Other than that, most things are calculated if not supplied. It is always best to have the full compliment of exif information if possible.
F number doesn't matter at all. The focal length will be calculated no matter what. The entered value just gives us a good starting place. Accuracy of the final product won't be affected.
The bottom line is we don't want anyone touching their tags. Tag values that are inconsistent with the imagery does more harm than good.