For 1 and 2: AFAIK Heroku already has three layers, the Nginx routing mesh, Varnish cache and your dynos. Everything is already handled for you so if you have two dynos, the traffic will rotate between the two. You don't need a load balancer for Heroku. It's built-in.

Note: Varnish isn't available on the (standard) Cedar stack. Here's how Heroku suggests you approach cacheing:

https://devcenter.heroku.com/articles/http-caching

To answer my own question, there is a free addon called pgbackups that will do this for you, and Heroku automatically load balances between dynos.

It's possible to run multiple server processes of a single Rails application on a single Heroku worker to gain some additional concurrency capacity, obviously with some caveats and limitations:

http://michaelvanrooijen.com/articles/2011/06/01-more-concur...