Wasn´t he just holding back forcing Angron to act?

Doubt that.. Since he did not know that Gulliman was coming to Nuceria.

Pretty sure you are correct there.

From Lexicanum:

"...and the Lord of Macragge battled first Lorgar then Angron. However this had all gone according to Lorgar's plan, who used the immense rage Angron was emitting to begin a Khornate ritual which transformed the World Eaters Primarch into a monstrous avatar of Chaos"

At least it was planned...

In the book he does not realize this until after Angron joins the fight

Horus would have flattened Guilliman. With or without the assistance of the dark gods.

I'm honestly surprised that Angron didn't manage to kill Guilleman in their early confrontations during the Heresy, i mean, Angron should definitely be one of, if not the strongest primarch in a one versus one scenario. I have no doubt that, since Khorne excels in one-on-ones, Angron could beat a lot of the primarchs out there. The 'Night of the Wolf' happened when Angron had just finished the Ghenna Scouring, and his men were exhausted and weakened slightly from all their massacreing, it would be interested to see an actual battle between both forces at full strength, although I personally think Angron would win, even though I love Russ.

Let´s face it...the power of the Primarchs may switch from HH novel to HH novel...right now, Vulkan is the strongest AND he is immortal (and as of now, the only thing probably able to kill him is not at the disposal of any other Primarch). That´s a tough nut to crack.