Does the Roc Warrior use his mobility ability to "flyaway" before he is countered or does he have to accept being countered and then "flyway"?

Counters go off before the Roc can move again.

Thanks!

Given that "attack" is a complete step attack/counter/advance the mobility effect would occur post step.

If I'm not mistaken, what I understand from Mobility is that it triggers right after the WHOLE attack step has been completed.

So any flying units that were ordered that turn, can move only after ALL attacks have been completed.

i.e. they move right before the upkeep phase.

That my reading too: you resolve all your attacks in sequence (and all the eventual counters) and then you move the Roc Warrior(s), immediately beforming the Upkeep step.

I agree with the above - Mobility triggers at the end of the entire Attack Step, after ALL attacks (and counters) have been resolved.

Can mobility be used after the Roc Warrior had to retreat (by a counter-attac)? I don't see anything that does forbid it to me...

I agree. After retreating, your Roc should still be able to move with Mobility when the attack is complete.