Any project using renv will, by default, use its own project-local environment. Those project libraries are isolated both from each other, and also from the "default" R library paths (e.g. the user or site libraries). If you are using renv with a project, you can rest assured that any packages you install or uninstall will not effect users outside of that project.
I believe the answer is yes. We have a number of users who are productively and effectively using renv in production environments. If you're considering adopting the use of renv as a standard within an organization, we strongly recommend reviewing the documentation at Introduction to renv • renv.
You do not -- renv is designed to work standalone without RStudio. However, when using renv within RStudio, some minor UI affordances will be made to help make renv easier to use (e.g. the packages pane will show packages installed in the project library, and also compare those versions with those in the lockfile.)