Is a decision from an Israeli ‘Sha’ari’ court awarding a divorcee compensation from her ex-husband, according to terms in her divorce agreement concerrning her maintenance, valid in Germany?

In principle, yes, providing the court ruling bears the ‘Apostille seal’ proving its genuiness, and the Shaa’ri (Moslem religious) court in Israel had jurisdiction to pass judgment, and did so according to law, and the ruling is final. However, you would be advised to check this out with a family law expert in Germany.