What is the best way to deal with game state and changing levels?

I am a professional programmer but new to game development and Unity. I am working on a simple arcade game where currently I have a single scene which will be used for all my game levels (levels differs only on how/what enemies are spawned). Currently I only have the 1st level and I need to decide how to deal with level changes, game state and main menu. Therefore I have a couple of question:

1. When changing level should I use SceneManager to reload the scene or write some custom script to remove all spawned game objects and reset level?
   

2. How to deal with game state? I have seen a couple of approaches: normal c# singletone (https://www.packtpub.com/books/content/creating-simple-gamemanager-using-unity3d), unity specific singleton (http://clearcutgames.net/home/?p=437), preload scene (https://stackoverflow.com/questions/35890932/unity-game-manager-script-works-only-one-time/35891919#35891919) but I don't know which is considered as best practice.

3. Should main menu be a separate scene or it's better to build GUI on the top of my single scene?

4. When having multiple scenes and singletons using DontDestroyOnLoad how to deal with testing individual scenes? Each scene should have the objects added or there is some way to load them form the first scene even if I want to check some other?

Thank you in advance.

Best regards Lukasz, I am a professional programmer but new to game development and Unity. I am working on a simple arcade game where currently I have a single scene which will be used for all my game levels (levels differs only on how/what enemies are spawned). Currently I only have the 1st level and I need to decide how to deal with level changes, game state and main menu. Therefore I have a couple of question:

1. When changing level should I use SceneManager to reload the scene or write some custom script to remove all spawned game objects and reset level?
   

2. How to deal with game state? I have seen a couple of approaches: normal c# singletone (https://www.packtpub.com/books/content/creating-simple-gamemanager-using-unity3d), unity specific singleton (http://clearcutgames.net/home/?p=437), preload scene (https://stackoverflow.com/questions/35890932/unity-game-manager-script-works-only-one-time/35891919#35891919) but I don't know which is considered as best practice.

3. Should main menu be a separate scene or it's better to build GUI on the top of my single scene?

4. When having multiple scenes and singletons using DontDestroyOnLoad how to deal with testing individual scenes? Each scene should have the objects added or there is some way to load them form the first scene even if I want to check some other?

Thank you in advance.

Best regards Lukasz