(1)

\begin{align*} \mathrm{comb}_{T}\left(-x\right) & =\sum_{n=-\infty}^{\infty}\delta\left(-x-Tn\right)\\ & =\sum_{n=-\infty}^{\infty}\delta\left(-x+Tn\right)\cmt{n\rightarrow-n}\\ & =\sum_{n=-\infty}^{\infty}\delta\left(x-Tn\right)\\ & =\mathrm{comb}_{T}\left(x\right) \end{align*}

(2)

\begin{align*} \mathrm{comb}_{T}\left(ax\right) & =\sum_{n=-\infty}^{\infty}\delta\left(ax-Tn\right)\\ & =\frac{1}{\left|a\right|}\sum_{n=-\infty}^{\infty}\delta\left(x-\frac{T}{a}n\right)\\ & =\frac{1}{\left|a\right|}\mathrm{comb}_{\frac{T}{a}}\left(x\right) \end{align*}