Increasing the maximum walking distance isn't that easy because it is not limited by a specific number of hexes. Instead it is limited by the amount of memory that is reserved for pathing. That memory is in fixed locations and if you would tell the pathing function to use a larger area then it...
It's actually pretty easy if you let sfall do the work for you: The value at 0x51871C seems to be 1 if a conversation window is open. So if one modifies sfall to present an upscaled 640x480 rectangle instead of the whole surface if this value is 1 then the desired effect is archived. This only...
I see you did not get the dialog picture right.
At 0x44AF25 (US 1.02d) there is PUSH EAX. If you modify the code so that it always pushes 0x280 (640) instead of eax then it will work.