If you destroy a objective with target of opportunity but still have a unit with objective damage ready can you choose another objective to attack? Also if you a have two units one with a objective damage and one without and attack a objective does the unit without a blast damage get a focus token given it did not engage any units during the engagement?

Your units are attacking the original objective and you cannot decide to allocate the damage elsewhere during the engagement.

And all units participating in the engagement are focussed to strike, even if they strike nothing.