Jelly,  52  51 bytes

^{Jelly has not got one regex(s) atom}

Ṫ
Ñ;”s
Ṫḣ-3
UṪw“)s(”⁼1
e€“µḣ⁴µuʠg*»Ḳ¤Ḣ‘×Ç‘
⁶;ḲÇĿ2ƤK

A full program accepting a string (using Python formatting if multiline or containing quotes) and printing the output.

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How?

Ṫ - Link 1, tail: two words (list of lists)
Ṫ - tail

Ñ;”s - Link 2, tail and replace last three chars with an 's': two words (list of lists)
Ñ    - call the next link (3) as a monad
  ”s - literal 's'
 ;   - concatenate

Ṫḣ-3 - Link 3, tail and remove the last three chars: two words (list of lists)
Ṫ    - tail
  -3 - literal minus three
 ḣ   - head from index (1-indexed and modular)

UṪw“)s(”⁼1 - Link 4, tail ends with "(s)"?: two words (list of lists)
U          - upend (reverse each word)
 Ṫ         - tail
   “)s(”   - literal [')', 's', '('] - that is "(s)" reversed
  w        - index of first sublist equal to that or 0 if not found
         1 - literal one
        ⁼  - equal?

e€“µḣ⁴µuʠg*»Ḳ¤Ḣ‘×Ç‘ - Link 5, categorise: two words (list of lists)
             ¤      - nilad followed by link(s) as a nilad:
  “µḣ⁴µuʠg*»        -   compression of string "a 1" + word " an" + word " one"
            Ḳ       -   split on spaces = ["a", "1", "an", "one"]
e€                  - exists in (that) for €ach (word in the two words) 1 if so, else 0
              Ḣ     - head (the result for the first of the two words)
               ‘    - increment
                 Ç  - call the last link (4) as a monad - i.e. f(two words)
                ×   - multiply
                  ‘ - increment - so we have: 1 for ["1", "blah"],
                    -             2 for ["blah", "blah(s)"] or 3 for ["1", "blah(s)"]

⁶;ḲÇĿ2ƤK - Main link: list of characters, the string
⁶        - literal space character
 ;       - concatenate (place a space at the beginning as we want to inspect pairs)
  Ḳ      - split on spaces (giving an empty list at the start)
     2Ƥ  - for all infixes of length two:
    Ŀ    -   call the link at the given index as a monad:
   Ç     -     call the last link (5) as a monad
       K - join the result with spaces
         - implicit print