In ESS/Emacs, how can I get the R process buffer to scroll to the bottom after a C-c C-j or C-c C-r

Probably a bunch of ways to do it. In my .emacs.d I have

  (setq comint-prompt-read-only t)
  (setq comint-scroll-to-bottom-on-input t)
  (setq comint-scroll-to-bottom-on-output t)
  (setq comint-move-point-for-output t)

You might also be interested in this code, originally from Felipe Csaszar, which lets you do what you ask and a few other nice things besides.