As Evan points out, "modern" technology (including Littelmann paths and canonical bases) provides an improved way to think about tensor product decompositions for simple Lie algebras. But your Theorems 1 and 2 could also be understood in purely classical terms, though I'm not sure how far anyone looked at these. The assumption is that the classification of simple Lie algebras $\mathfrak{g}$ over $\mathbb{C}$ is in hand and we ignore rank 1. Here $\mathfrak{sl}_{n+1}$ has Lie type $A_n$ with $n > 1$.

Theorem 1 then is basically a case-by-case observation, using the known description of root systems as in Bourbaki (or for exceptional types more explicitly in Springer's table here).

Then Theorem 2 is just counting the number of summands isomorphic to the adjoint module in the tensor product of this module with itself. Here the module has highest weight equal to the highest root, which I'll call $\gamma$ (Bourbaki denotes it by $\widetilde{\alpha}$). From the case-by-case study one knows (as indicated) that the multiplicity of the adjoint module here, or equivalently the dimension of the Hom space, is at least 2 for type $A_n$ and at least 1 for other types. So the remaining problem is to make these bounds exact.

It's a standard (but intricate) classical problem to work out such tensor product multiplicities for arbitrary finite dimensional highest weight representations. However, the basic approach (going back to Brauer's *Comptes rendus* note in 1937 and further developed by Klimyk) typically involves a huge amount of cancellation along with a summation over the entire Weyl group $W$. But the idea is quite simple: in our case, add to $\gamma$ the weights (= roots along with 0) of the second factor in the tensor product, each counted with its multiplicity: 1 for each root, $n$ for 0. This gives the full list of irreducible summands with their multiplicities, but only if you transform each non-dominant weight in the list into the (shifted) dominant Weyl chamber via the dot-action of $W$ given by $w \cdot \mu = w(\mu+\rho)-\rho$. When this unique weight is dominant in the strict sense, attach the sign of $w$ to the resulting multiplicity in the tensor product.

Brauer's method in fact implies here that we get at most $n$ occurrences of the adjoint module as summands of the tensor product. So the problem is to reduce this using Theorem 1. This one does directly using the reflections $s_i$ corresponding to the simple roots along with standard root system information such as the fact that $s_i \rho = \rho - \alpha_i$ and that $s_i \gamma = \gamma$ whenever $\alpha_i$ is orthogonal to $\gamma$. Thus $s_i \cdot (\gamma -\alpha_i) = \gamma$ (and the sign is $-1$) in the orthogonal situation. There are more details to fill in, but the point is that it's all fairly straightforward and classical even though not transparent.

UPDATE: This question led me to consult a specialist (code name SK), who recalled a more general theorem but not its source. I asked about that here. Just now my
consultant has retrieved the original source in a 1996 paper by R.C. King and B.G. Wybourne here. Like most of King's other work, the paper involves classical Lie theory of interest in mathematical physics. The proof relies on techniques such as Schur functors and plethysm but not on Littelmann paths, etc. (The article itself seems to be restricted to those with library subscriptions.)

As I noted in my question, Allen's theorem 1 translates into the more general hypothesis: the highest weight of the adjoint representation (i.e., the highest root) involves in each case just one or two fundamental weights, being orthogonal to the others.