gistfile1.el

(defun my/fix-space ()
  "Delete all spaces and tabs around point, leaving one space except at the beginning of a line."
  (interactive)
  (just-one-space)
  (when (looking-back "^[[:space:]]+") (delete-horizontal-space)))
  
(defun my/delete-backward ()
  "When there is an active region, delete it and then fix up the whitespace."
  (interactive)
  (when (use-region-p)
    (delete-region (region-beginning) (region-end))
    (my/fix-space)))  

;; Same as above, but using (interactive "r") to get the bounds of the region    
(defun my/delete-backward-2 (beg end)
  (interactive "r")
  (when (use-region-p)
    (delete-region beg end)
    (my/fix-space)))

;;    
;; Variant that deletes backward one char unless the region is active:
;;

(defun my/delete-backward ()
  (interactive)
  (if (use-region-p)                  ; IF

    (progn                            ; THEN
      (delete-region (region-beginning) (region-end))
      (my/fix-space))

    (delete-backward-char 1)))        ; ELSE
    
;;
;; Improved version of the last example that handles a prefix arg the same way as `delete-backward-char.
;; - If there is no region: delete backward one character, or N characters with a prefix argument.
;; - If the region is active and the prefix arugment is 1 (or not set), delete the region and fix space.
;; - If the region is active you can use a prefix arg to delete a certain number of chars instead of the region.

(defun my/delete-backward (n)
  (interactive "p")
  (if (and (use-region-p) (= n 1))
      (progn
        (delete-region (region-beginning) (region-end))
        (my/fix-space))
      (delete-backward-char n)))

I think this should do it:

(defun my/fix-space ()
"Delete all spaces and tabs around point, leaving one space except at the beginning of a line and before a punctuation mark."
(interactive)
(just-one-space)
(when (or (looking-back "^[[:space:]]+")
(looking-at "[[:punct:]]"))
(delete-horizontal-space)))