Thank you, Alex, for the suggested edits.  I moved the enum for cache types into the main struct for multiple_cursor_cache, and also moved the sections that apply to w->mc_elts into the MC_CACHE case instead of the CH_CACHE case.  Attached is the updated diff for the new cache management design.

struct multiple_cursor_cache
{
  ptrdiff_t allocated;
  ptrdiff_t used;
  struct items
  {
    int x;
    int fx;
    int y;
    int fy;
    int hpos;
    int vpos;
    int wd;
    int h;
    int cursor_type;
    int cursor_width;
    struct RGB
    {
      double red;
      double green;
      double blue;
    } foreground, background;
    bool active_p;
    int glyph_flavor;
    bool enabled_p;
  } *caches;
  enum type_of_cache
  {
    NO_CACHE,
    MC_CACHE,
    CH_CACHE,
    FC_CACHE
  } cache_type;
};

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

> Date: [04-20-2019 10:17:01] <20 Apr 2019 11:17:01 -0600>
> From: Alex Gramiak <agrambot@gmail.com>
> To: Keith David Bershatsky <esq@lawlist.com>
> Cc: emacs-devel@gnu.org
> Subject: Re: Universal functions to manage multiple window caches.
> 
>* * *
> 
> Looks about what I expected, yeah (you could also have the enum be an
> element of each cache type instead of it being global, but it's up to
> preference). Though is it your intention to be using w->mc_elts in the
> CH_CACHE case instead of w->ch_elts?