You should briefly think how opam does updates currently: a repository on GitHub where developers submit their pull requests. This won't change too much.

Guarantees are: if a new package from a new developer is submitted, basically none. The signing of delegation and developer keys (by repository maintainers) enables a web-of-trust style system: once package `foo` has been claimed by the well-known (and signed) developer `alice`, there's no way that `eve` will successfully update `foo` (without getting `alice` key, and also then need to go via GitHub and repository maintainers).