WTC: First plane was too high, Second plane was 18 minutes late, not level or centered. The goal was surely 20,000+.The first plane into the WTC came over Manhattan. It couldn't have hit the WTC much lower without being in danger of hitting other buildings first.
I think the delay was planned so that live TV news cameras would se it. If that was the plan, it worked perfectly.
As for where it hit, the resulting fires weakened columns on two sides of each building. It's more likely for that to occur where they hit than it would be if they had centered the hits. Going through the center means the plane would have had to penetrate the core to get to the other side. By hitting to the side and cutting diagonally across a corner, they were sure to damage two sides of the outer skin. The outer skin was the main load bearing steel on those towers.
In my opinion, this delay was a classic terror-bombing tactic, as used in WWII. First strike brings out the rescuers and firefighters. Follow-up strike kills them.