Ok. I am probably over emphasizing the risk of zero alkalinity to be safe.
Low pH can work, but the big question is if it will remove the stains without damaging anything else.
The lower the pH goes, the better and faster it will work but the risks also go up as well.
If I was to do the zero alkalinity, I would go right to zero TA and not lower.
I would probably only wait 8 hours before deciding to bail out if it wasn't helping.
I would also watch any metal carefully and bail out if it looked like it was changing color.
Some light niches are copper.
Copper will go from a dull brown to a bright shiny copper color like a new penny when it begins to be etched by low pH.
In any case, hopefully, the sulfamic acid will work and you won't need to try the low pH.