KABUL, Afghanistan — President Ashraf Ghani has promised a complete overhaul of Afghanistan’s government to root out corruption and incompetence, but after three months in office, three missed deadlines and countless promises, he and his election-rival-turned-deputy have yet to appoint a single Cabinet minister. The repeated delays have meant that Ghani is single-handedly grappling with a dizzying array of challenges — from a revitalized Taliban insurgency to a reset of long-fraught relations with neighboring Pakistan — and have fueled speculation that he and Chief Executive Abdullah Abdullah, the equivalent of his prime minister, are still at each other’s throats. A wide range of sources close to both men, including Afghan associates, officials and Western diplomats, say the two have amicably shared the task of selecting potential ministers. [...] the Taliban remain a potent threat, having carried out a string of high-profile attacks in the capital in recent weeks and caused record casualties among Afghan security forces this year, killing 5,000 troops and police.