1. Before multicore got to this PR stage it went through two phases of detailed review by the core team. A summary of this is on November's Multicore Monthly: https://discuss.ocaml.org/t/multicore-ocaml-november-2021-wi... . The tasks coming out of that which are marked post-MVP will be follow-up PRs before the 5.00 release.
2. That is unclear at the moment. There's a lot of useful history in those commits (which link out to issues and PRs on ocaml-multicore's repo) but at the same time, it also includes a lot of experiments that were ultimately backed out.