Is it possible to get the playlist associated with the metadb_handle_ptr list that is output by a context menu command? Besides the method of comparing metadb_handle_ptr locations with those in all the playlists, which has the problem of more than one playlist being matched due to duplicates, is there a better way?

You should check the caller parameter, if it is contextmenu_item::caller_playlist, then assume the items are the selection of the active playlist. If it is a different caller, do not assume the items are from any specific location - playlist or otherwise.