I'm currently working on plethysm, i.e. the character of the composition $S^\lambda(S^\mu(V))$ of the Schur functors $S^\lambda$ and $S^\mu$ on a complex vector space $V$. We note this character $s_\lambda[s_\mu]$. One must know that the character of $S^\mu(V)$ is the Schur function $s_\mu$.

We want to find the irreductible representation of $S^\lambda(S^\mu(V))$, or equivalently, to write $s_\lambda[s_\mu]$ in a sum of Schur function. There is very little that is known about this, and this operation is rarely mentionned in the litterature I found.

I have worked with plethysm mostly in the context of a "math research traineeship", so I skipped some details of all this theory to emphasize on the use of the computer in pure maths.

I have 3 questions about all this and I don't find references:

1) I know how to compute plethysm in terms of power sum symmetric functions, but why is it defined this way?

2) What is known or conjectured about operations on plethysms that are Schur-positive (i.e. having only nonnegative coefficients when expressed in terms of Schur functions), like the Foulkes' conjecure, that said that $\forall a,b \in \mathbb{N}, \ a \leq b, \ h_b[h_a] - h_a[h_b]$ is Schur-positive, where $h_n = s_{(n)}$, the schur function indexed with only one part?

3) Where can I find clear versions (and with the use of modern notations) of the Thrall's proofs of $h_2[h_n]$, $h_n[h_2]$ and $h_3[h_n]$?

Thanks in advance for short explanations or for references!