Unity3D code in Visual Studio

I'm using old Unity3D 5.1 without any plugin for Visual Studio. Because I'm in the middle of the project now, don't want neither to upgrade Unity3D nor to install VS plugin for Unity3D, just in case. Because I don't like Mono Development as a default editor, am just thinking of switching the option to Visual Studio in the Unity Preferences.

So I want to know in advance if there would be any problems when writing and debugging code with Unity3D 5.1 no plugin installed.

I know I can write .NET 3.5 code for Unity3D in VS for compatibility, but what I'm really worrying about is Unity3D code in Visual Studio. Unity3D scripts use some syntaxes from .NET 4.0, such as Named Parameter and Optional Parameter. That means I cannot set .NET Framework version to 3.5 in Visual Studio, right?

Do I have to set to .NET 4.0 in Visual Studio for those Unity3D scripts?

Will there be any problems if I compile managed DLLs(for Unity3D) with .NET 4.0 in VS?