Think I do know the answer to this put want to put it here anyway:

If Gondorian Spearman or a character with Spear of the Citadel defends against Smaug, will his ability prevent the dealing of 1 damage from taking place?

Along those same lines, if a character with Dwarrowdelf axe attacks, is Smaug immune to that 1 extra damage the axe does after the attack?

yes to both!

I am always confused by 'immune to player card effects' and to what extent this applies. Does this mean an Erebor Battle Master doesn't get his bonus attacks? This is a player card effect. What about Gandalf's ability to apply 4damage?

I can understand attachments like Forest Snare not being permitted but how far does this immunity go?

Fisrt: attacking an Enemy (through the rules, not a card effect) and calculating the damage isn't a card effect. Erebor Battlemaster's attack strength isn't affected by Smaugs ability, because the effect affects the Battlemaster, not Smaug.

The immunity goes up to denying any effect that would apply to Smaug: Boromir won't deal damage to Smaug after sacrificing him. You can't chose him as target for an effect, like Gandalf. If there would be only Smaug and another Enemy, you couldn't play Small Target, because there's only 1 valid target. You can't play Feint or Quick Strike on Smaug.

Actually you can play quick strike.

The Hero is the target.

Actually you can play quick strike.

The Hero is the target.

Immune to Player Card Effects

Cards with the text “Immune to player card effects” ignore the effects of all player cards. Additionally, cards that are immune to player card effects cannot be chosen as targets of player card effects.

Quick Strike

Action: Exhaust a character you control to immediately declare it as an attacker (and resolve its attack) against any eligible enemy target.

As I read the card and rules, Smaug is the target Quick Strike.

Edit: clarified comment

I´m a bit confused. Are you trying to say that when a card is immune to player card effects, that also means it cannot be chosen as the target of an attack?

I´m a bit confused. Are you trying to say that when a card is immune to player card effects, that also means it cannot be chosen as the target of an attack?

No.