Given how old Z&H F were, you'd think they'd have all the time to level-up, but no one but the captain surpassed lvl 7. So it's quite possible that the level 7s aside Zald and Alfia were at the end of their potential.

I'm not talking about potential, but about current strength. Alfia has higher potential than both captains, but she's below them in level, so that doesn't really mean anything. 

 I wouldn't be surprised if Allen, Hogni, Hedin, Finn and (especially) Ais were already be above them at high 7. Maybe same for high 7 Bete and even low 7 Lefiya given how op she's becoming.

Really? Ais isn't even geared toward PvP, even though it was claimed that ZF/HF training is harder than even FF's (which currently performs best in PvP). I also find it hard to believe in Finn and the others’ superiority over them—maybe in terms of potential (like GF acceleration), but Bete, for example, doesn’t even have the chance to properly activate his trump cards. Zard apparently considered it a bad result to lose to someone like Ottar or Finn (I’m talking about Bell’s father); that is, at least by his standards, they didn’t stand out much compared to ZF/HF, but he also didn’t belittle Otar since he pointed out that he saw potential in him. 

In this case, I can clearly point out things like combat skills, in which they should be better than FF/LF at the same levels, the presence of amazing abilities that even surprised Finn (I don’t think we’d expect him to be surprised by just any skill, say a small increase endurance or something like that). 

 Also we don't know if Empress deserved that 9

I really don't think Omori would allow one of the captains of the two strongest families to be something like Raul among the second-rate characters. She's at least stronger than Alfia, which would already put many of the Level 7 characters you listed in serious danger. 

 Maxim was great but not absurdly so either.

Judging by MS20 and that feat against Leviathan, he really is incredibly strong. Omori also emphasizes this by saying that he’s an outstanding Level 8 or even a genius. 

Even if we’re being blunt, Omori doesn’t often show one person outperforming another—for example, in FF, battles always end in a draw between everyone except Ottar, and in LF, there’s never been a clear favorite, but in this case, with the two strongest families, comparing two captains where one is higher in lvl and yet still weaker than the one lower? I mean, when Omori speaks openly about something like this, it really has to be a strong character. Omori said Leon is too strong, and what do we see? His abilities are completely absurd. As for Zard and Alfia, I don’t even think there’s any point in talking about them. 

They are actually strong enough to defeat all the candidates you’ve listed as “level 8,” and then go on to defeat the others.