So i was wondering : can Son of Arnor engage goblin sniper if there are other enemies in the staging area?

Yes. Son of Arnor force one engagement, and goblin snipper only avoid the optionally engagement in an specific situation (when there are more enemies in staging).

PD: you can use sneak attack + son of arnor when is your turn to attack, and you kill directly (like knight of minas tirith)

great, thx mate