A keeping test will only tell us whether the concentrate keeps, but won't tell us much about the effectiveness of the citric acid chelating agent against hard water. I'm quite confident that citrate won't tie up iron enough to render it ineffective against the Ascorbate.
Here's my take:
- You can find chelate stability of various citrate salts here.
- The strongest competitors for Ca2+ in any film dev will be sulfite and carbonate. Carbonate also competes for Mg2+.
- With estimates for ion concentration and the numbers taken from the linked pages above you can calculate whether Ca2+ or Mg2+ will likely precipitate.