Booking the Kandy to Ella train — how do I actually get a seat and how far in advance do I need to book?

The Kandy to Ella train is supposed to be one of the most scenic train journeys in the world but I've heard it's extremely hard to book. I don't understand the booking system.

1. How far in advance do I need to book to get a seat in 2nd or 3rd class observation car?
2. Can I book online from outside Sri Lanka and if so, what website actually works?
3. What is the difference between 1st, 2nd, and 3rd class on this route?
4. Is 2nd class observation worth paying more than 3rd?
5. What happens if I can't get a reserved seat — is standing/unreserved worth it for the scenery?
6. What time of day is best for the light and the views?
7. Is the full Kandy–Ella route the best or is just Hatton–Ella good enough?

I want to plan this properly — it sounds like the experience can be ruined by poor planning.