regarding catchers that learned to catch in the minors:
that happens but it is probably much more common that a HS catcher gets moved to another Position. yes to Play in MLB you Need to hit but the requirements to catch in the big leagues are very high and most fringy catching prospects that can hit get moved (some to protect their Body like harper but most because they just can't catch in the Majors).
if MLB would be "bat first" with catchers they would just put failed outfield prospects behind the plate instead of having a backup catcher with a .600 OPS.
of course glove only doesn't work in MLB, but catchers are the weakest hitting Position and the most likely Position for a weaker bat (note that a weaker MLB bat is still an amazing batter that likely batted well over 400 in high School) to make an MLB roster. there have been a few exceptions like Vmart and Napoli but generally you have to be extremely polished to make the Majors and catchers on average are on the older side of most rookies compared to other positions.