Thanks you for your answers.

I have played with code and added hundred (not really) printf and Log to understand how flow of events works. I think this is doable.

a) How I should implement it?
X11 is handling events like this: 
switch (event.type)
  case KeyPress:
     Game.DoKeyboardInput

And XInput2 (MPX) is handling it:
switch(event.xcookie.evtype)
  case XI_KeyPress:
    Game.DoKeyboardInput

So should it be
#ifdef MPX
switch(event.xcookie.evtype)
#else
switch (event.type)
#endif

#ifdef MPX
  case KeyPress:
#else
  case XI_KeyPress:
#endif

    Game.DoKeyboardInput

Now they are guaranteed to be synced.
Another way is something like:
static void _internal_keypress(something) {
    Game.DoKeyboardInput
}

#ifdef MPX
switch(event.xcookie.evtype)
  case XI_KeyPress:
    _internal_keypress(something);
#else
switch (event.type)
  case KeyPress:
    _internal_keypress(something);
#endif

b) Should it be compile or runtime decision?
It can be easily checked with
  /* Check for XI2 support */
  int major = 2, minor = 0;
  if (XIQueryVersion(display, &major, &minor) == BadRequest) {
    printf("XI2 not available. Server supports %d.%d\n", major, minor);
  }

c) What is preferred way to send patches?
I have used to use git. I happily make smallest (=atomic) possible commits (I have never used hg).