I like your point 1 - more of a quality control type of thing. It also implies that the taper need not be dramatic, just slightly larger than the the machining errors. I may incorporate that idea into future mods. I initially had the idea you meant a more severe taper. Luckily my design largely rules out windway height variance (relies on sheet manufacturers tolerances), and the width cutting is done with a knife under reasonably favourable conditions so I am largely confident in the accuracy of the width.
I am not so sure of point 2. Are whistlers really aware of wind speed per se, or do they just respond to back pressure? I would favour the latter view which implies that only the area at the narrowest point will matter, but as I am not an experienced player (far from it) please take this opinion with a grain of low sodium salt substitute.
I wonder if a choke at the exit end of the windway would achieve the same effect? Many of my fipple blocks are wood, so if I glued a matchstick at the end of the block would I get the wanted effect or would I get too much turbulence? I must have a try, after my current project- a flute walking stick (Bamboo is such a wonderful medium 
)