Using 'equation split', how can I ensure that only the last equation is numbered?

You seem to be wanting to load amsmath with the tbtags option:

\documentclass[twocolumn]{article}
\usepackage[tbtags]{amsmath}

\begin{document}

\begin{equation}
\begin{split}
a & = b + c \\ & = c + d \\ & = d + e
\end{split}
\end{equation}

\end{document}

(Note: twocolumn is just for making a smaller picture.)

The tbtags option for amsmath does just that:

\documentclass{article}
\usepackage[tbtags]{amsmath}
\begin{document}
\begin{equation}
\begin{split}
a & = b + c \\ & = c + d \\ & = d + e
\end{split}
\end{equation}
\end{document}

Note that if you'd want to place equation numbers to the left (with option leqno), they'll go to the top of split automatically.

Instead of a {split} environment, use an {aligned}[b] environment; no other changes needed.

\begin{equation}
\begin{aligned}[b]
a & = b + c \\ & = c + d \\ & = d + e
\end{aligned}
\end{equation}