Stalin wrote fairly extensively on Leninism and other forms of socialism including Trotskyism and Anarchism. In these works he argued heavily for Marxism-Leninism. Stalin implemented the state seizure of the economy, both with agriculture and the heavy industries, which Leninism calls for.

Of course it is impossible to know exactly what is opinions were, and obviously they might have changed throughout his life. But, he did follow the center tenants of Leninism, even if he went about it in a strong handed way.

Probably, but he may not have all the way throughout the period of his rule. As Nikita Khruschev put it, referencing Stalin's cult of personality: "It is impermissible and foreign to the spirit of Marxism-Leninism to elevate one person, to transform him into a superman possessing supernatural characteristics akin to those of a god." Which is exactly what happened in the soviet culture after Stalin's 50th birthday, all the way up until his death when Khruschev soon after came into power and tried to denounce Stalin's status as a god. If Stalin in 1929 and onwards really believed in Leninsm-Marxism, it is odd that he would not try to stop this elevation of status. It didn't seem like he was particularly thrilled about this status either, though, since he on several occasions tried to make the people praise some of the other soviet leaders instead of himself. I think he was very conflicted in this area.