Can't move files to trash without any permission issues

This is a bug in nautilus, since the same doesn't happen in Trusty (as you found out by yourself); the explanation for this behavior might be that nautilus always tries to delete the files reached through the /home path as being physically located in the /home partition even when they're reached through a symbolic link to a folder physically located in another partition: in this case "trashing" the file like usual fails because the target file cannot be be moved to the user's Trash, hence the prompt to delete it immediately instead.