I had forgotten about this. I gave it another go the other day and its still not working as expected.
Now it seems to be a plugin priority issue between this plugin and transmission. The torrents are added to transmission but it seems to be as magnets (instead of using the torrent file downloaded by this plugin), and then it crashes because its trying to add a torrent file that was removed (because convert_magnet thinks it was already added?). I tried to mess around with plugin priority, but had no luck.
The plugin is downloading the torrents correctly from the magnet URIs, and i suppose i could added them from another task using the filesystem plugin, but i think there should be no need for that.