Normally C++ files should be handled as c++14 unless you have explicitly specified a different standard in your Makefile settings. Could you please double-check your CFLAGS/CXXFLAGS? You can see the effective IntelliSense flags via View->Clang IntelliSense Status->Project Status. Does the CXXFLAGS field specify -std=c++1y?
The handling of .h files depends on your project type. When you open a .h file, VisualGDB first checks if any of the source files includes it. If yes, it opens the .h file in the context of that source file. If no sources include the header file, it simply counts the C and C++ sources in the project. If there are 50% or more C++ sources, new .h files are treated as C++ includes. Can you please check how many C and C++ sources are in your project?
The command for switching between source and header files will be fixed in the final 5.0 release.