You're missing the point and forgetting what his sister keeps saying. Inaho is "special" and successful against Martians because he is both insanely reckless and doesn't lose his shit under ridiculously stressful situations...and frankly, Inko isn't much different.
Inko is a little more cautious, but neither of them realize how insanely dumb they're being. The adults perform worse because they're not dumb teens. Teens are always reckless, it's part of being a teen. The difference is that occasionally Inko realizes that she's about to die and freezes up, usually cringing (adorably) in her cockpit.
Inaho just keeps his cool. He logically or rationally realized early on that if he doesn't act he's gonna die anyway so...why the hell not?
Contrast the two of them to Rayet. She's far more like the adults and carefully plans and plots revenge, though still has that impulsiveness of being a teen. It screws her over time and time again. She's way more competent than either Inko or Inaho, but she's cowardly and cautious and it bites her in the ass. She's reckless when she has plenty of alternatives (killing the princess), and methodical when she doesn't have any options and can't really afford to be. Rayet is far and away the cause of most of the implausibility in the series. Unlike the others, she's not trained as a pilot and none of the students or teachers recognize her, but they frequently let her pilot anyway, and there's never been any basis for why she isn't completely clueless getting into cockpits the first few times.
As for Saazbaum's mecha's weaknesses...Inaho already fought them all. They knew the trick with the barrier and they understood that they could easily counter-attack the joints of the flying arms because he was fighting them close up with them like an arrogant fool. I suppose it was weak that the creators never bothered with a more original mecha for him, but he didn't really have to "figure anything out" there. Saazbaum had no idea Inaho was the one doing all the killing so far. Hubris brought down every single one of the Orbital Knights prior to that as well.
Saazbaum was the dumbass in that fight. He went inside the castle, rendering the drones that let him see outside the barrier worthless (resulting in needing to frequently drop the barrier), and used the flying arms up close where they could attack the body. Had he kept them outside, he would have defeated them easily with his mech's speed and agility alone.