I don't really see the Historia of being stronger than the originals, because of what Gildarts said about God Serena not having any living magic. Though even if they were, it doesn't really change my position. Irene enchanted Neinhart with so much power than Brandish's magic can no longer affect him. This was so that he can personally hunt down Erza by himself. There's no way he's weaker than just one of his Historia.Broadly I agree with your assesment, but I think you're wrong about scaling Historia to alive characters
I think Historia Ul is far stronger than alive Ul, because it was powered by Neinhart.
Neinhart's historia are powered by his magic power, which was why Azuma, Ikaruga and Kyouka low-mid diff'd Erza in a 3 vs 1. I don't think the excuse of Erza being not being her peak holds here, because she was in a far worse shape when she faced Eileen.
Erza faced Neinhart after a day of being healed by Wendy from Ajeel's fight.
Neinhart fucked her up far worse than Ajeel did, and Erza got healed again for a day. Then she faced enchanted Alverez soldiers who injured her by quite a bit, and then Natsu & Gray fucked up both her arms. Even if Wendy healed her arms before Eileen fight (which I believe she did), they were not completely healed and those injured hands were shown in fight against Eileen. Erza, by faar, was more injured and tired against Eileen than against Neinhart.
So Neinhart's 3 historia, combined, at least scales above that.
Neinhart is definitely (IMO) top 6 among Spriggans in terms of pure magic power, even if he's fodder in stats.
I think this is how Neinhart spread his magic power among Historia, in Hargeon. With the generic rule : put enough magic power in the historia to match or overwhelm the opponents. The only Historia in Hargeon, that I don't see being it's peak, is that of Hades. The rest of the Historia were SURELY buffed IMO
In Alverez reunion after Eileen's country shaping magic, I'm pretty sure than Neinhart could've made Bradman or Wahl above their peak if he really concentrated on creating only one of them; and Serena at probably 70% of his full power if Neinhart created only Serena.
I agree that Neinhart is above most of the Spriggans when it comes to MP, though that's largely due to Irene's enchantments. I don't rank his durability that low either. He's no worse than Brandish.

