As someone who played college baseball, I'd like to think I know what I'm talking about here.
That said - the question posed is really kind of silly. You can either throw a ball really hard or you can't - yes, with good training and proper mechanics you can add a few MPH to your fastball - but you can't just take someone off the street and make them or develop them into someone who can throw an MLB-caliber fastball (90mph+).
It isn't like working out - where you could technically take someone off the street, have them train, and they get in much better shape - you aren't going to 'make' major league pitchers.
As for the discussion with position players pitching - these guys have been playing baseball since they were little kids - yes, pitchers don't bat as often any longer, but many of them, in fact most of them, were star players in HS and college both pitching and hitting.
Also -Tim Wakefield is a knuckle-ball pitcher - how hard his fastball is outside the discussion.
Ankiel, in particular - could have been a center fielder or a pitcher - he chose, and the Cardinals chose, to start him on the mound - Josh Hamilton was a great pitching prospect as well - going back a bit - John Olerud - the guy who had to wear a batting helmet in the field (precaution), was one of the dominant pitchers in the NCAA when he was at Wash St.
Finally - hitting a major league pitching - the comment is often made that it's the hardest thing to do in professional sports - that is really impossible to quantify, but in terms of reaction time and hand-eye coordination needed, it's got to be up there - returning serves in tennis, goalies stopping shots in hockey, I'd imagine there are a few others with similar reaction requirements - and none of them are easy.
Finally - as for the guy talking about Independent league pitching - there are very few guys in those leagues throwing over 90 mph, and I'm talking about all the Independent leagues, combined, across the country. If you can throw a baseball 90+ mph and have some idea of where it is going, major league teams are going to find you and give you a chance in the low minors. The Independent leagues are mostly made up of guys not quite good enough to play minor league ball - and for pitchers that mostly means they don't throw hard enough.