I have a eight hour layover at night between Frontier flights. Can I stay overnight in the Frontier terminal?

Yes you sure can could catch the train downtown and come back for around $20 round trip

yes

I have a 16 hour layover. Are there restaurants in the airport

Sure

Is there public transportation from the airport to downtown Denver?

Most nights you can. But you'll want to call ahead to verify.

Yes you can stay overnight

Call ahead