The powers of a matrix form a linear recurrence sequence (with characteristic polynomial equal the minimal polynomial of the matrix). Hence, the question essentially is about the periods of such sequences in rings $\mathbb Z/p^k\mathbb Z$. There are tons of literature on this topic. In particular, your questions may be answered in Chapter 3 (Linear Recurring Sequences Over Artinian and Finite Rings) in the monograph Linear recurring sequences over rings and modules but I did not check that.
ADDED. On a brief look, it seems that the following answer to (ii) is implied by Proposition 17.2: $$s_A(p^k) \leq k \cdot \mathop{\rm deg} \mathrm{minpoly}(A).$$