ep_queue_size and ep_diskqueue_items are aliases to the same statistic - the number of items waiting to be persisted to disk. The only reason there’s two names is historical, there was an effort to give more consistent / user-friendly names, but we didn’t want to delete the “old” stat in case some users were depending on it.
disk_write_queue - this is a compound stat generated by ns_server (hence why you won’t find it listed in the stats.org link above). I just checked the ns_server source and it’s the sum of ep_queue_size and ep_flusher_todo. As per stats.org, the definition of ep_flusher_todo is:
ep_flusher_todo - Number of items currently being written