A couple of things about Torrence: he hasn't played organized ball in, what?, a month or more? Plus, who was he facing? College pitcher? Pitcher repeating the league? If he's willing to give up because of a bad start... Plus his signing bonus payment is broken down so that if he quits too early he won't get the full amount.
There is plenty of good news in the org: Pence, Patton, Paulino, Towles, Flores, Manzella, Iorg before freak injury, James, Bogusevic, Salamida, Perez, Gervacio, Johnson, Torres, Einertson, Van Ostrand, Parraz, Sapp, DeLome, Dixon, some of the Venezulelan players are adjusting to the US ok. Plus several of the top high school draftees haven't signed yet, Dinelli hasn't played yet.
The Astros draft the best player available, especially in the top 7-10 rounds regardless of pitcher or position player and tools. Too, they're not going to reach just to get a guy with a particular tool. Top of the order kind of guys in the system: Anderson, MRod, Flores, Ash, Sutton, Moresi, and several drafted this year. Some are better prospects than others, but the Astros do have them.
Manzella is as you say. He's a major league shortstop. He's not yet a major league hitter. His chances in the majors are completely dependent on his ability to hit enough. Rest assured the Astros are doing/have done whatever they can to make him a better hitter. If you look at the top 10 from this year you'll see we include a note that the Astros are looking at him as a possible Everett replacement down the road.