Difference between MATLAB mex API to MATLAB engine API

I am having diffculties in getting (and, thus, setting) graphic objects' properties via matlab's engine API: I'm always getting NULL pointer in the following code (Am using R2015b on windows 8.1):

    #include <engine.h>
    #include <matrix.h>
    #include <mex.h>
    #include <mat.h>
    int main()  
    { 
      Engine *MATLAB;
      if (!(MATLAB = engOpen(NULL))) 
      {
     //exit failure etc.
      }
       engEvalString(MATLAB, "clearvars;close all;x=linspace(-pi,pi);figure;h=plot(x,sin(x),'o-b','LineWidth',2.5);");//OK!! got the plot on a new figure
       const mxArray *ph = engGetVariable(MATLAB, "h");//OK!!
       const char *cname = mxGetClassName(ph);// OK!!!: got cname = matlab.graphics.chart.primitive.Line
       size_t ind = 0;
       const char *Prop = "LineWidth";
       mxArray *p = mxGetProperty(ph,ind,Prop);//bummer !!! - p is always NULL!!
       return 0;
    }

Now, when writing the equivalent code with mex API, everything works perfectly as follows:

1: Am running the next MATLAB's script:

    mex getMex.cpp;%compile getMex.cpp (with VS 2010 Ultimate), see code below
    clearvars;close all;x=linspace(-pi,pi);figure;h=plot(x,sin(x),'o-b','LineWidth',2.5);%OK!! got the plot on a new figure
    LineWidth = getMex(h);% OK!!  LineWidth = 2.5

the getMex.cpp source file:

    #include "mex.h"
    void mexFunction(int nlhs, mxArray *plhs[], int nrhs, 
                 const mxArray *prhs[])
    {
      //some input/output checks here
      mxArray *p = mxGetProperty(prhs[0],0,"LineWidth");  //OK!!! - not NULL
      double *p2h=mxGetPr(p);//OK!!! *p2h = 2.5
    plhs[0] = p;
    }

While debugging both the mex code and the engine code in VS 2010 I saw that the same dlls were loaded exactly.

What is the problem with my engine API code?

What am I missing here?