1. Tarkin's ability to reduce objectives strength, is that always in effect while he is in play or does he need to be attacking that objective?

2/ Cards that can be played from your hand in certain circumstances, such as during combat as a reaction, do they still need to be paid for with resources?

Tarkin's ability is in play as long as he is in play. If an effect must be triggered by his participation in an engagement, the text will read, "While attacking / while defending / while participating in an engagement"

If your card, like Ackbar or Backstabber says you can be played from your hand during an engagement, you still have to pay for them. However, if the text reads, "put into play" that means it's free!

Thanks!