[Erlangen]

Sorry I miss your reply. I create opam switches to have different versions of compiler, or allow libraries that conflict to live at different branches.

1. Dune is a relative new build system for Ocaml. You just write a configuration file, and dune will handle switches automatically. "It also supports multi-context builds, such as building against several opam roots/switches simultaneously. This helps maintaining packages across several versions of OCaml and gives cross-compilation for free." See https://opam.ocaml.org/packages/dune/

2. Yes. You can can multiple versions library at different switch.

3. I am not sure.