Alphabet Between Encryption
Pyth, 38 bytes

M|strFGjHGsm?&gt;Fd_g_d\Lgd?qFd\Z\R.:rzZ2


Demonstration.

M|strFGjHGsm?&gt;Fd_g_d\Lgd?qFd\Z\R.:rzZ2
                                          Implicit: z = input(), Z = 0
                                          g is a function of a 2 letter string
                                          and an insert letter.
M                                         def g(G, H): return
    rFG                                   Take the string range between the
                                          letters in G.
   t                                      Remove the first letter to get just
                                          the letters in between.
  s                                       Concatenate.
 |                                        Logical or (if nothing in between)
       jHG                                H.join(G)
                                  rzZ     Convert input to lowercase.
                                .:   2    Take 2 letter substrings.
           m                              Map d over the substrings
            ?&gt;Fd                          If the first character of `d`
                                          is greater than the second,
                 _g_\L                    g(d[::-1], 'L')[::-1]
                      gd                  else, g(d,
                        ?qFd              if the letters of d are the same,
                            \Z            'Z'
                              \R          else 'R')
          s                               Concatenate and print.