- Your link says these are elements commonly found in steel, not that they are all required. In fact it says of phosphorus and sulphur that they are generally undesirable.
- We don't need to make a steel sword, an iron sword could do.
Either way you would definitely need carbon, but as you say that's pretty easy. I don't think any of the other elements are absolutely required.
Looks like a destroying angel (e.g. Amanita virosa) to me. This and the death cap together account for the vast majority of mushroom poisonings in the world. Cooking it will not destroy the toxins, nor will acid. Symptoms tend to appear 5-24 hours after eating, too late to pump the stomach. Half a mushroom can be enough to kill you.
I don't recommend going out to pick mushrooms unless you know what you're doing. If you do, stay away from the white ones. You can still get terrible stomach cramps and diarrhea from other colors of mushrooms, but the white ones have the most dangerous species.