Is there any trick to save diamonds to exchange wolffangs pendants or to show up all 4 cards without..

No, if you are given an officer spot, and the buffs that go with it, you need to spend the 4800 diamonds per week to buy two cards (via the wolf fangs) per day. You add two of your own cards per day as well, for four in total.

These cards provide the bonuses that bring missions to you, and from which you get your class points for the week.

It's a responsibility, you need to meet that requirement in exchange for the buffs and to support your state.