This won’t do what you expect because you’re checking for a tag with underscores but then setting a tag with spaces. To check for a tag with spaces, you need to use the $get() function instead of the percent signs. For example, something like:
$if($get(Media Release Date),,
$set(Media Release Date,%date%)
$set(date,%originaldate%))
As for what tags Picard uses and how they are mapped, check out the Tag Mapping appendix in the Picard User Guide.