@Tourma A launcher is essentially another app that just acts differently.

1. No, there is no need for that. However, this may affect device navigation controls - for example, on Samsung's OneUI interface (a skin on top of Android), you cannot use gestures with a custom launcher app, unless you root your phone. Other brands like Google Pixel may allow you to do so, albeit there may be some small differences when it comes to the app opening/closing animations, just minor things that you may not care about.

2. No, it's a separate app. The stock launcher is still there, and can still be switched back to from Settings > Default apps > Home app (or Launcher)

3. Not really, no. Configurations between the home screen of your stock launcher and the custom launcher(s) that you use should be separate, so there shouldn't be any issues there.

4. Absolutely not.