Does this look like a stain from metals?

[waterbear]

A couple of thoughts on your problem....you said the heater was shot. If it is a copper heat exchanger and it has become damaged by improper water chemisty then it is possible that your staining is from copper. Copper will not always stain blue. The Jack's Magic website stain reference library page
http://www.jacksmagic.com/page.cfm?id=387
shows an example of a copper stain that is brown and looks like an iron stain (3rd stain on the page). Ascorbic acid can cause this type of copper stain to turn greyish (as also shown in the picture on the Jack's Magic page, Stain Treat#1 is listed as propriatary ingredients on the MSDS but I suspect it is either ascorbic or oxalic acid).
I don't know if you really need to drain and refill twice. If it were my pool I would treat the stains and then drain and refill the pool to get rid of the high CYA level and any metals in the water, then rebalance the water and test it to see if there is any copper or iron still present. If the level of metals left in the water is not that high I would just use a sequerant on a regular basis to keep them from restaining and keep my pH on the low side, 7.2-7.4. To treat these stains I would first get a Jack's Magic stain ID kit to find out exactly what I am dealing with since the vitamin C seems to produce a strange greyish color (which can indicate a copper stain) and there is some possiblity that the damaged heater could be the source of the copper.
As an alternative to the Jack's Magic products (which work very well but are expensive and can be lenghty procedures for some types of stains) you might want to try Proteam's Metal Magic first. They have an easier test and treatment but it is not always as effective. How to do their 'sponge test' is on this page
http://www.proteampoolcare.com/prod_...SpongeTest.pdf
and they state on this page that the treatment is not affected by CYA levels.
I have used both Proteam and Jack's magic products and they both work well. Jack's Magic has an extensive line of stain treatments that are very effective but they are not easy to use and can take several weeks for certain types of stains. Proteam's Metal Magic is not always as successfull but it is a less expensive option and easier. If it were my pool I woud try it first if the 'sponge test' had favorable results. Once the stain has been lifted it means the metal is now in solution in the pool and that would be the time to drain and refill. You will solve your high CYA problem and the metals in the water in one shot.
One more thing you might want to try on the stain to determine if it is metal or organic is to put some dry acid in an old sock and hold it on the stain to see if it lightens or changes in any way. If this is effective then acid washing (if the pool is plaster) might be an option (or last resort) also if the above two ideas don't work.