Amitrole is slightly toxic to various species of freshwater fish$^1$. However, it's quite water soluble and should not be difficult to remove from most surfaces with simple warm-water washing.
Ammonium Thiocyanate is particularly water soluble and should also readily be removed via warm-water washes. Thiocyanate ion has lower solubility with some other cations that would be present, but in general, thiocyanates are quite water soluble. Thiocyanate ion has been shown to be toxic to fish at the tens of mg per liter level, however, $^2$.
Simazine has a very low water solubility, on the order of 5 mg/L (4-5 orders of magnitude less soluble than the other compounds) but has been shown to be lethal to fish at levels on the order of its limit of solubility$^3$. I strongly suspect that residual simazine is thus to blame for the persistent toxicity to your fish.
Simazine is about 100 times more soluble in isopropyl alcohol (rubbing alcohol) than it is in water. I suggest obtaining some 99% isopropyl alcohol and scrubbing/soaking all of the decorative material that you value, as well a the tank itself, thoroughly rinsing the alcohol off with warm water, then drying completely. There is no reason to believe that vinegar would be useful in this capacity.
Anything relatively cheap should probably simply be replaced, particularly difficult to clean items like driftwood, high-surface area gravel or sand, and of course all of the filter media. My biggest concern of the items you mentioned is the crushed coral. If it were me, after the cleaning I would return everything to the tank except the crushed coral and once the water is ready, test it out with a couple fish. If things seem OK for a while, then I would set up the tank with everything including the crushed coral.
As a former aquarist, I understand how much work is involved with my suggestions. I strongly believe, however, that you have identified the culprit in the fish die-offs and that the procedure above has a good chance of resolving the issue. Please don't hesitate to ask for any clarifications in the comments below.
- US EPA. 1984 (March). Amitrole: Pesticide Registration Standard and Guidance Document. Office of Pesticides and Toxic Substances, US EPA, Washington, DC.
- Speyer, M. R. and Raymond, P. (1988), The acute toxicity of thiocyanate and cyanate to rainbow trout as modified by water temperature and pH. Environmental Toxicology and Chemistry, 7: 565–571. doi:10.1002/etc.5620070705
- WSSA Herbicide Handbook Committee. Herbicide Handbook of the Weed Science Society of America, 6th Ed. WSSA, Champaign, IL. 1989