Yes, fish are in danger with those levels. However, if you can keep it down with changes you'll be fine. Your cycle is not quite complete, but the presence of nitrate is a good sign.
I wouln't so much about bio filtration. You're set up quite well, especially for the stock you have. Keep in mind that your undergravel filter is using all of your gravel as biomedia..and they were used successfully for many years before anyone dreamed of paying for overpriced ceramics.