Anyone know where I can find a specification for the ex-mode substitute command? It's not clear to me what emacs command, if any, it corresponds to. I have trouble with some of it, eg I never seem to be able to successfully match start/end-of-line. It'd be really nice just to have a definitive reference for the syntax.

Or, as a fallback, does anyone know how I can get help for an ex command?

No sooner said than solved. Mapping of keys in ex mode to commands is in https://github.com/magnars/.emacs.d/blob/master/site-lisp/evil/evil-maps.el

And a related question: is anyone aware of another regex substitution command built into spacemacs that stays closer to standard regex syntax?

Bingo. To make :s aka evil-substitute use vim-style regexes, set the variable evil-ex-search-vim-style-regexp to a non-nil value. Works a treat.