Does the delay in completing a marriage after performing الاستخارة (istikharah) and things becoming easy, along with the presence of obstacles preventing its completion, mean that there is no decreed share (نصيب)? And does praying for the facilitation of marriage contradict contentment with God's decree?

There is no harm in repeating the istikhara prayer, as it is a repetition of supplication (du'a), and repeating supplication is legislated in Islam. Supplication itself is part of Allah's divine decree. One of the most important conditions for marriage is the guardian (wali). If the woman's guardian cannot be present for the عقد (marriage contract), he can appoint someone to perform the marriage in the suitor's country, or send an authorization to someone in her country to marry her. However, it is not permissible for a woman to appoint herself as her own guardian, according to the preponderant opinion.