Reducing factoring prime products to factoring integer products (in average-case)
This is not really an answer, yet it sheds some light on the difficulty of demonstrating such reductions.



The problem can be summarized as follows:
Let $\mathcal{A}$ be an algorithm which solves the INTEGER-MULT problem with non-negligible probability. Let this probability be at least $n^{-c}$, for some constant $c \in \mathbb{N}$ (when the size of input is $n$). Prove that there exists a PPT (probabilistic polynomial-time) algorithm $\mathcal{A}&#39;$ which, uses $\mathcal{A}$ as subroutine, and solves an instance of the FACTORING problem of size $n$ with probability at least $n^{-d}$, for some constant $d \in \mathbb{N}$.

Consider an algorithm $\mathcal{A}^*$ which solves the INTEGER-MULT problem if and only if the input $N$ has the special from $N = PQR$, where $P$ and $Q$ are primes of size $n/4$ and $R$ is a prime of size $n/2$, and otherwise fails. Moreover, on input an integer $N$ of the above form, it outputs $PQ$ and $R$. We first show that $\mathcal{A}^*$ has a non-negligible probability of solving the INTEGER-MULT problem. To this end, it suffices to find the fraction of $n$-bit integers of the special form, as this fraction bounds the success probability of $\mathcal{A}^*$ from below.

By prime number theorem, the number of primes whose size is $k$-bits is:

$2^k / \ln(2^k) - 2^{k-1} / \ln(2^{k-1}) = \Theta(2^k / k)$

Therefore, the fraction of $n$-bit integers of the special form is:

$\frac{\Theta(2^{n/4} / (n/4))^2 \cdot \Theta(2^{n/2} / (n/2))}{2^{n-1}} = \Theta(n^{-3})$

which is non-negligible in $n$.

Therefore, one should be able to prove the existence of a PPT algorithm $\mathcal{A}&#39;$ which, uses $\mathcal{A}^*$ as subroutine, and solves an instance of the FACTORING problem of size $n$ with probability at least $n^{-d}$, for some constant $d \in \mathbb{N}$.

IMHO, this seems a very difficult task, since $\mathcal{A}^*$ only (partially) decomposes integers of the special form, and there does not seem to be a way to use it to decompose integers of the form $PQ$.