WI Luther did not challenge key aspects of Catholic doctrine, and remained committed to political reform of the Roman Church only?
As is well known, Luther's initial concerns rested with Rome's abuse of indulgences as income generators for the papacy. Also high among his concerns was the abuse of Mass stipends for votive Masses on behalf of beneficiaries. This latter action proved an important source of income for monasteries, and in fact remains an income source for some monasteries. (Do know, however, that nowadays many priests will happily say their daily Mass gratis for your intentions -- just ask.) The sale of indulgences is now strictly prohibited and Mass stipends are regulated, but these reforms appeared long after Luther's criticisms.
These criticisms, as well as discomfort with clerical celibacy and its abuses, were not Luther's alone. However, Luther eventually denied core doctrines of Catholicism in two prominent areas. He sharply criticized Catholic eucharistic theology, replacing the idea of propitatory sacrifice with a reliance on the "promise" of divine presence through the repetition of scriptural passages only. He also rejected the role of theological and historical development in the beliefs of the Church, preferring to affirm beliefs only found in scripture. His denial of the Canon of the Mass and its sacrificial character, as well as sola scriptura, struck at the heart of Catholic ideas of salvation and grace.
Would Luther still have rejected reconciliation with the institutional Church if his criticisms called a council and political-financial concerns were corrected? Or was his eventual rejection of Catholic sacramentality and development of doctrine the flaming sword between him and the Catholic theological-doctrinal tradition?