For running J!5.1 you must https://fabrikar.com/forums/index.php?wiki/update-from-github/ or include the new file manually https://fabrikar.com/forums/index.php?threads/joomla-5-1-and-fabrik-cannot-find-files-error.54473/post-285151 See also Announcements
Please see our announcement here.
Did you try the csv list plugin?
There are "Import row", "After import row" "Import start" and "Import complete" hooks.
I don't know what your will get there in details, I assume some var_dumps will help.
$db = JFactory::getDbo();
$query = $db->getQuery(true);
$query->update('fab_matches')->set('player_id = NULL')->where('player_id = 0');
$db->setQuery($query);
$db->execute();
Try in "Import row" or "After Import row" with something like"Code to run before each rows is imported. Modifying $formModel->formData['yourtable___yourelement'] will modify the data ..."