The quantity of hypochlorite or chloramine in the water is already low when it comes out of the tap (typically around 2 ppm). Since you have already boiled your water, you have significantly de-gased and decarbonated the water. The addition of CRS should not have much alkalinity to act on when you're adding it this late in the process.
Considering you are already going to the trouble of boiling your water, I'm not sure that you need to use the CRS since the bicarbonate content (which creates alkalinity) has already been reduced to between 60 and 80 ppm. I wrote a short guide on decarbonization by boiling here:
You would not produce gaseous chlorine gas when you add a metabisulfite compound. It actually changes the chloramine or hypochlorite ions back to chloride ions.
Download Bru'n Water to learn more about brewing water chemisty. There is a section on water knowledge in the program. You can download the program from the link in my signature line below.