Just one advice then. If you decide to start deleting data.
Make a separate file for players (there are 600K of them) and delete per nation. If needed use one file per bunch of nations (Italy will be one file, there about 60K of Italians in the database)
Make sure whenever you delete largest amounts of data, to NOT save. You will thank me later.
If you want to delete nations, you can do so by removing the continent, but.... important warning, nations cannot be deleted without removing them from their respective international competitions.
So....., remove or rebuild international competitions first*
* With regard to deleting nations. What you can do, and what I have done for previous large projects is deleting all international competitions. Saved that into a separate "delete" file. After that, whenever I would rebuild a competition (in another file), I would undo the delete of the competition in the delete-file. Works quite handy.