The way you can tell if the stain is from metals is to use some ph down, or crush some vitamin C tablets up and put them in a sock, or stocking and rub them on the stain, if the stain goes away, it is a metals stain. If it stays, then it is from something else.
It is fine to aerate and treat a stain too - it is good, because you have to keep the ph low to do both.
I can't tell you about the mottled look of the plaster, but if they are metal stains they will come off with the vitamin C or ph down. If they are metal stains, this treatment should help - if it doesn't they you can get rid of them by using ascorbic acid (which is vitamin C). This will take the stains off the surface of the pool, and the Jack's Magic is a sequesterant which will hold the metals in suspension. The sequestering agent (Jack's Magic) will sometimes help lift off newly formed stains, but if they are too embedded they will have to be removed by using a stain treatment, such as ascorbic acid.
Sequestering agents can sometimes lower ph - I know that Jack's Magic does.

Hope this helps, feel free to ask any questions you may have.