Answer 1

There's also MonoDevelop, SharpDevelop and Xamarin. I don't know how lightweight they are though. Your best bet is to do some research on the web

Answer 2

For PC, you have to go with Visual Studio 2017. For your Android tablet, there aren't many (if any) good offline C# IDEs and compilers

Answer 3

i just downloaded D<oder on my tablet. I think this is about as good as it gets on a tablet. PC, just use Visual Studio, or Atom