If a system with a Base gets attacked, the defender picks his objective, lets say Ion Cannon Defense and the attacker has skilled spacer token... Can he change it to ANY other campaign objective?

What if this is done on a system that has a Base, Independent Station and Asteroids? Same question. Can the Attacker, with skilled spacer token change it to ANY campaign objective?

Only the defender get any benefit from spacers.

The only objectives spacers can affect are the 3 'fleet' objectives.

Special assaults, base defense and location objectives are unaffected.