All groups are linear algebraic over some fixed field $k$.

I believe that it is true that, in characteristic $0$, if $G'$ is a reductive subgroup of $G$, then there is a $G'$-invariant complement to $\operatorname{Lie}(G')$ in $\operatorname{Lie}(G)$.  I guess this is nearly a consequence of complete reductivity, but I'm not sure how to ensure rationality of the complement if some isotypic component for the adjoint component of $G'$ on $\operatorname{Lie}(G)$ is neither contained in, nor disjoint from, $\operatorname{Lie}(G')$.

In positive characteristic, this can fail.  I think, but haven't checked, that the embedding $\operatorname{PGL}_2 \to \operatorname{GL}(\mathfrak{pgl}_2)$ in characteristic 2 is a counterexample.

Now, there are other phenomena that behave well in zero or large characteristic, but that fail in small characteristic, like the existence of Jacobson–Morosov triples.  In this setting, we have McNinch's theory of optimal $\operatorname{SL}_2$-homomorphisms, of which it is my understanding that they often provide a substitute for JM triples that allow the use of many of the classical techniques.

My question is whether there is some similar substitute for the existence of complements to reductive Lie subalgebras in small positive characteristic.  A specific example that would work for me is if it were known that, whenever $G'$ is a reductive subgroup of $G$, then $\operatorname{Lie}(G')$ surjects onto $\mathrm N_{\operatorname{Lie}(G)}(\operatorname{Lie}(G'))/\mathrm C_{\operatorname{Lie}(G)}(\operatorname{Lie}(G'))$.  (This follows from the existence of a complement in large characteristic.)