I have disable/commented out QwtMathML to compile the examples ( but i think this my environment issue)
It makes sense to do this if you are not interested in the MathML stuff - but it is not necessary. If you had to do this to build the examples successfully you might have a serious problem on your side, that should be fixed before you continue.

1) Is there any special patch/config for qwt and mingw64 to work properly? all i did was edit qwtconfig.pri to my needs ,qmake (64 bit made qmake) and mingw32-make (comes from mingw64).
I'm not aware of any reason, why Qwt code should depend on 32/64 bit and I heard about successful 64 bit builds with MSVC. But I never tried mingw64 myself.

2) I have built my app during the qwt 5.2.0 days. Now i noticed that my apps are complaining about qwt_data.h is not available anymore. I was wondering whats the story about the deprecation of QwtData and what is its replacement?
Check the examples and the documentation.

Uwe