1. The <PERSON> hierarchy is a hierarchy of grammars described by a given formalism. The other grammar descriptions that you are giving (attribute grammars, ...) are not part of that formalism so they are outside the <PERSON> hierarchy (and if I'm not mistaken are able to recognize some languages of higher level in the hierarchy while being unable to recognize other languages at a lower level).
2. Context-sensitive grammars are not at the top of the <PERSON> hierarchy, unrestricted grammars are.
3. 312
Context-sensitive grammars can generate what you are asking for, but it's painful to write. Here is a grammar which generate a s followed by a binary number, followed by as many p as determined by the binary number.
\begin{align} S & \rightarrow \text{s} \; C \ \text{s} \; C & \rightarrow A \ \text{s} \; C & \rightarrow \text{s} \; B \; \text{p} \ \text{s} \; C & \rightarrow \text{s} \; B \; C \; \text{p} \ A \; C & \rightarrow A \; D \ A \; D & \rightarrow B \; D \ B \; D & \rightarrow B \; \text{p} \ B \; D & \rightarrow B \; C \; \text{p} \ B \; C & \rightarrow B \; E \ B \; E & \rightarrow X \; E \ X \; E & \rightarrow X \; \text{p} \ X \; E & \rightarrow X \; C \; \text{p} \ A \; X & \rightarrow A \; F \ A \; F & \rightarrow B \; F \ B \; F & \rightarrow B \; A \ B \; X & \rightarrow B \; G \ B \; G & \rightarrow X \; G \ X \; G & \rightarrow X \; A \ \text{s} \; X & \rightarrow \text{s} \; B \; A \ A & \rightarrow 0 \ B & \rightarrow 1 \ \end{align}
Yes that's a lot of rules (I may have missed some shortcuts, but on the other hand I may have missed some cases where unwanted strings are generated which could be avoided only with a more complex grammar). Some explanations:
* I'm using $A$ and $B$ for temporary place holder for $0$ and $1$ because context sensitive rules do not allow to modify terminals. The last two rules replace them by the terminals they were acting for.
* the first three rules are pretty obvious.
* the fourth is starting the count
* the next four are transforming a count ending by a $0$ in a count ending by a $1$ and a $\text{p}$
* the next four are transforming a count ending by a $1$ in a count ending by a $X$ (meaning a carry to be propagated) and a $\text{p}$
* the last seven rules are doing the carry propagation. 904