The lesson suggests a sequence: intend what is right, act accordingly, correct errors, and seek steadfastness to the end. A good beginning does not remove the need for self-examination later.
الْأَعْمَالُ is nominative because after إِنَّمَا the nominal clause retains its normal subject form. The noun after the preposition بِـ in بِالْخَوَاتِيمِ is genitive.
Daily worship, trustworthy conduct, and the rights of others all require ongoing attention. That is a practical response to a report emphasizing the ending.