Does stunned unit can perform counter-attack immediately (by paying 2 lore to "unstun") or it has to wait to player order step ?

Stunned units cannot counter. From the reference guide, pag. 9, under Stun:

A player cannot move, attack, counter, or retreat using a stunned unit.

Denis, I'm not a 2E player yet, but for similar situations in 1E one would have to wait until ordering that effected unit before paying the lore to remove the effect. I assume that 2E is the same, and one would not be able to immediately pay 2 lore to "unstun" in order to be able to counter.

EDIT: read through the reference book, and yes indeed, have to wait until the order phase to order that unit and then pay to remove the stun token.

Another question about stun.

If golem rolled stun and retreat, would both events happen, or I must choose one as stunned unit cannot retreat?

Dice are committed during step 4 of the Combat resolution (you can refer to pag 2 of the Reference Guide). Any effect that is not cause damage, cause retreats, gain lore is immediately resolved. This implies that Stun is given during this step. Retreats are resolved during step 8, hence later: Stun is active, retreat cannot be resolved due to stun and damage is inflicted.

Hope this helps!

Yes! Thank yuo!