The attack done with the Great Yew Bow is resolved immediately or it is resolved during the normal "player resolves attacks against his enemies" step (so after all the enemies have attacked)?

It is a combat action, so you can trigger it when there are windows for actions. For example, after players has attacked, or before..., or even before enemies attack! But only in combat phase.

It is a combat action, so you can trigger it when there are windows for actions....

But only in the combat phase That's the sad story.

Is the same true for Forth Eorlingas?

Combat Action: Each Rohan hero can be declared as an attacker against enemies in the staging area this phase.

But not for Dúnhere?

Dúnhere can target enemies in the staging area when he attacks alone.

Yes, for both