When considering the stability of transition metal chelates, it is not just pH and photostability that should be considered but ion exchange between solvated ions such as Ca2+ and Mg2+ and the transition metal ion to ligand bond in the chelated compound. There are lists of relative stabilities on the web.
I keep trace mixes such as Fe and other trace elements as chelated compounds in aqueous solution in dark bottles but without any other chemicals (a tiny amount of KHSO4 to stabilize pH excepted). When added to the tank I would expect Fe and other chelated metals ions to come out of chelation and be available to plants as simple solvated ions. I use Sulufeed Fe13EDTA and TMAG to provide iron and other trace metals, the latter is longer available but it has been replaced by a better product. Many are available with different chelatating compounds and as potassium salts instead of sodium salts and in small Kg amounts. I have, what I have, and then they were only sold by the sack when I and Adam were lads.
In hard water, Fe and other transition metal ions will be replaced from chelation by solvated Ca an Mg ions. The half life of the simple solvated Fe and other transition metal ions in a bare dark container will be pH dependent. However we aim for these simple salts to be absorbed by our plants and this should be the dominant factor in determining the fate of such metals. If not biologically absorbed their usual fate will be as a complex mix of oxides, hydroxides and phosphates, typically not biologically available. Over more than a ten year period I have never noticed iron precipitation in my tanks (moderately hard water) but in the field I have found plenty of gelatinous iron precipitates from iron rich rocks and from mine outflows into receiving waters with both high and low calcium contents. [Some locations in the UK have complex geologies]. Since going 'high tec' I cut 50:50 with rain water to give moderately soft water but I add a little magnesium sulphate to push the hardness up a little.
The EI dosing regime takes this into account with alternate dosing days, thus giving plants the opportunity to absorb different nutrients over a 24hr period. I always assume that Fe and other transition metals will only be available for about 24 hours. Hence regular dosing with dilute solutions.