Backhand, 100 - 95 = 5

M#$%&"()*+H-./0e1234l5678l9:;<o=>?@,ABCD FGIJWKLNOoPQRSrTUVXlYZ[\c^_`b"dfgh]ikmn'pqst!uvwxryz{}a|~jE

Try it online!

Note that the version on TIO is missing a couple of instructions (notably '), so I've copied the interpreter here. This program works because Backhand executes every fifth instruction thanks to the first M (it's actually a little more complex than this), so the actual code is more like:

M    "    H    e    l    l    o    ,         W    o    r    l    c    "    ]    '    !    r    a   j 

However, since you can't remove the spaces inbetween without breaking the flow of the program, you can replace them with whatever you want. The final check is that the j jumps to the 10th character (H) to halt and output,, which is less feasible if you modify the program. It's possible you might be able to form a valid program from this mishmash, but it is unlikely, and I'm sure that there's another permutation that fixes that.

Explanation:

M                      Increase the step count from 3 to 5
 "Hello, Worlc"        Push the string to the stack
               ]       Increment c to d
                '!     Push !
                  r    Reverse the stack
                   aj  Jump back to the 10th character
  H                    Halt and output the stack