Can I pay to book a train more than 20 days in advance? What happens if train is then unavailable?

You could plan your trip now. While, all the tickets could only be purchased 20 days before departure. You'd better to prepare a plan B.

If you book via this site, you can request, and pay for, all of your trains in advance. Then, for each one at the 20 day point, they book the requested train for you, or a suitable alternative (approved by you). You should have no trouble getting the trains you want if you book now. I am currently in Beijing & have been here for 2 weeks during Spring Festival, the busiest time for travel & the hardest time to get tickets. I selected the trains I wanted, & made my requests/payment back in November, & I got every single one I asked for (5 in total). So I would recommend you use this facility.

To clarify ' in advance'....you can request/pay for trains more than 20 days ahead...you're effectively hiring them as your agent to sort it by doing this.