Iris

Is such an option viable?

Not really.

Griffins will readily attack any attacking unit twice, and with each attack, their strength will be 1.5 times greater until their turn comes (Combat Fury skill).

The ability only triggers on the first attack on the stack. After that, there will only be one counterattack.
However, it's easier to shoot down or eliminate griffins with magic.

They will feed on vampirism twice.

Vampirism can be removed in various ways.

I will take the following from the branches: Light, Darkness, Luck, Defense, and Leadership.

Two magic skills for a Knight with 0 Knowledge and Spell Power?
Without Attack and free Retribution?
Without Education in the late game?

Or is the standard build for Laslo, Axel, or Mivex still a more impressive force in the late game?

In my opinion, a standard build is a standard build, especially since the variability against different races is already high. You can't build up a stack of griffins, but you can with armored units or horses.