Yes, securing branches is very important, much like git-hooks if you are serious about the Git flow.

How do you specifically investigate the history with rebase and 10+ developers? Do you manually look into commits on the branch, or go by a single file history, or look at merge/pull requests?

Personally, I prefer looking at the context of a file change within a merge/pull request. The specific merged/pull request can be read from the merge commits. And in case a specific file history is of interest, I can backtrack from that file's associated commits.