Media Manager can only alter the Path field. You tell the tool what string in the path to search for and to replace it with another string. You can tell it to search and by leaving the replace field blank, it will erase the search string.
It can either remove the path set in Preferences from the record’s path, or add it.
I have never used the add media function. But from what I understand, if you have a
Births\ directory already in the records, the tool will find other files in the Birth folder and add the media record to Gramps if it does not already exist. But any added media record will not be added to any person or event. So you will have more work to do.
You can do this for all the media on you hard drive and then Merge (individually) with the media records created from the Ancestry import. Again, a lot of work depending on the number of records.
The Media Verify works off the media record’s checksum. You add a media record to Gramps, Gramps will set the checksum as meta information. So if you subsequently rename or move the file (after reorganizing the files on the hard drive) the tool will find that file again and automatically make the changes to the records. If you edit the raw file, crop or resize it, the Verify tool will no longer see it. It will report it as missing while at the same time reporting that the altered file is not in the Gramps records.
In Verify, there is a Generate option which will set/reset all the checksums. So once you have all the paths set, the Generate is a function you will want to run. But until the checksums are set, Verify cannot change the path information. It will just report there are all these media files in the directory that are not in Gramps but will not be able to add them. The Verify’s Fix option only works on files with known checksums.
As I said. The problem is that the Path information from the Ancestry import does not resemble the path structure to your files on the hard drive.