1) $4$ steps. First step is $12330000$, second step is $12331332$, third step adds (in order) to the fourth, seventh, fifth, and eighth, giving $12344355$, and last step adds (in order) to the fifth, seventh, sixth, and eighth, giving the desired $12345678$.

2) We notice that each step adds $9$, so since $2021$ is not a multiple of $9$, it's impossible.

3) We notice that $2145=3\cdot 5 \cdot 11 \cdot 13$. So at least four of the numbers are ones. We also know that the maximum number of ones we can have is $5$. So we just have to check the cases. Case 1: $1, 1, 1, 1, 3, 5, 11, 13$. These add to $36$ so we have to perform $4$ steps. But we know that with $4$ steps, the largest number we can have is $12$, contradiction. Case 2: $1, 1, 1, 1, 1, x, y, z$. Clearly $x, y, z$ cannot be $5$ or $3$, because there are at least $5$ numbers each added in the last three columns; thus the minimum is $10$. So we can only have $1,1,1,1,1,11,13,15$. But this is a contradiction as well because the eight numbers don't add up to a multiple of nine. Thus it's impossible as well