Parallax wasnt as strong as he was, but he is high herald at least. Thats all that his feats showed him to be.
And the second part is true.
And Galactus probably was slightly over the am, but he did take a galaxy destroying blast and still come out fighting, but after a long fight he would go down to galactus.
And I doubt that they would have time to expliot the red sun radiation weakness... Superman-prime would fighting waaay too fast. Terrax would be one-shoted almost immediately. The rest of the heralds would be either stalemated or beaten by the rest of the sc members, while Superman-prime flies by and speedblitzes them all and tips the scales if they aren't already that way. Then the am and al of his heralds dogpile Galactus. Thats how it would go imo.
Because Kyle was high herald, and he took on Hal and John at the same time. He was basically Kyle with an amp. And thats pretty powerful. People just see him as 'not high herald' because he didnt live up to what he was, so people dont bother looking at his feats.
Yes he will. If they're fighting people who are on their level or above, they wont have time to multitask. And yes, Superman-prime is that fast.
If Silver Surfer can figure out Gladiator's obscure radiation weakness the first time he saw him, I'm sure Galactus can find SMPs weakness that much faster.
Galactus is supposed to be freshly fed, so I could see him blasting all of them in the beginning and taking out Cyborg Superman and Sinestro in the first stages of the fight. Galactus's heralds are all the time to sacrifice themselves to save Galactus, so SS and co could split up to distract SMP and Parallax long enough for Galactus to go head to head with AM.
After Galactus finished off AM, the heralds would have most likely been killed by SMP and Parallax (with maybe the exception of Surfer, due to his recent upgrade), and Galactus can finish off the last two if he isn't 'out of juice' by then.