AutoHotkey (v2), input from stdin, output to stdout, 265 258 bytes

Since it should be a cat program in some situations, I suppose that stdin and stdout are required. A shorter script not fulfill it is also provided below.

FileAppend Sort(i:=FileOpen('*','4').Read(),'D+')=='6CO2+6H2O+light`n'?'C6H12O6+6O2':i=='r`n'?Format(s:="FileAppend Sort(i:=FileOpen('*','4').Read(),'D+')=='6CO2+6H2O+light{:c}n'?'C6H12O6+6O2':i=='r{1:c}n'?Format(s:={:c}{}{2:c},96,34,s):i,'*'",96,34,s):i,'*'

AutoHotkey (v2), input from command line argument, output to MsgBox, 231 225 bytes

MsgBox Sort(i:=A_Args.Has(1)?A_Args[1]:'','D+')=='6CO2+6H2O+light'?'C6H12O6+6O2':i=='r'?Format(s:="MsgBox Sort(i:=A_Args.Has(1)?A_Args[1]:'','D+')=='6CO2+6H2O+light'?'C6H12O6+6O2':i=='r'?Format(s:={:c}{}{1:c},34,s):i",34,s):i

Ruby -p, ⁹⁶ 87 bytes

eval q="$_=%w[6CO2 6H2O light]-$_.split(?+,3)==[]?'C6H12O6+6O2':/^r$/?'eval q=%p'%q:$_"

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The code is an extension of the Ruby quine

eval q="$><<'eval q=%p'%q"

The optional second argument to split is needed to avoid false positives on inputs like 6CO2+6H2O+light+. The check for an input of r uses a bare regex literal as a conditional, saving a byte over the more obvious $_==?r but provoking a warning from the interpreter.

Jelly, 77 70 bytes

“+“6CO2+6H2O+light r“C6H12O6+6O2“ḣ2œṣṢjɗḲi¥@ƭƒ³ị3ị,Ṿ;⁾0ị,⁾vƊƊƲ³ṭƊ”0ịvƊ

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A full program link taking a string argument and printing a string argument. This currently feels a little verbose for a Jelly answer!

Python 3, 164 142 bytes

m=r'm=%r;a=input();print("C6H12O6+6O2"if{"6H2O","6CO2","light"}==set(a.split("+"))else"m=r\x27"+m+"\x27;exec(m%%m)"if a=="r"else a)';exec(m%m)

Here is a Python quine that meets all the requirements.

Google Sheets, 109 bytes

=ifs(""=A1,,"6CO2+6H2O+light"=join("+",sort(tocol(split(A1,"+")))),"C6H12O6+6O2","r"=A1,formulatext(F1),1,A1)

Put the input in cell A1 and the formula in cell F1.

Uses a cat $0 type quine, so non-competing.

JavaScript (Node.js), 79 bytes

f=x=>x.split`+`.sort().join`+`=='6CO2+6H2O+light'?'6O2+C6H12O6':x=='r'?'f='+f:x

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Nothing special

05AB1E, 98 bytes

1"D34çýU'rQiXë“6CO26H2O“2ä'‡Žªœ'+ýIåi“C6H12O6+6O2"D34çýU'rQiXë“6CO26H2O“2ä'‡Žªœ'+ýIåi“C6H12O6+6O2

Outputs the C6H12O6+6O2 with two instead of one trailing newlines. Could be fixed in +2 bytes by adding “ after both C6H12O6+6O2 in the program.

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Explanation:

1                   # Push a 1
 "..."              # Push the program's source code
      D             # Duplicate it
       34ç          # Push 34, and convert it to a character with this codepoint: '"'
          ý         # Join the stack with '"'-delimiter
           U        # Pop and store this quine-string in variable `X`
'rQi               '# If the (implicit) input is "r":
    X               #  Push quine-string `X`
                    #  (which is output implicitly with trailing newline)
   ë                # Else:
    “6CO26H2O“      #  Push string "6CO26H2O"
     2ä             #  Push it into two equal-sized parts: ["6CO2","6H2O"]
       '‡Ž         '#  Push dictionary word "light"
          ª         #  Append it to the pair
           œ        #  Get all six permutations of this triplet
            '+ý    '#  Join each inner triplet with "+"-delimiter
    Iåi             #  If the input is in this list:
       “C6H12O6+6O2
                    #   Push string "C6H12O6+6O2\n"
                    #   (which is output implicitly with trailing newline)
                    #  (implicit else)
                    #   (implicitly output the implicit input with trailing newline)