why is it that any time i read about a nutrient (iron, magnesium, etc) i check my multi-vitamin and find that it's not even there? if these are so important, why I am taking 12,000% of my daily vitamin B and C and 0% of some other ones?
It's estimated that 200,000 Americans a year suffer from an iron overload which can cause all sorts of problems including damaging your heart. So it's not considered to be a good thing for a "one a day" supplement to habitually contain iron.
Many Americans are deficient in potassium and you might think, "cap up some KCl and take one a day". KCl can slow down your heart rate and reduce your blood pressure which is good to some extent but if you could buy KCl supplement that gave 100% of your daily potassium need some people would be dropping dead from an overdose of potassium (a potassium is part of the usual lethal injection cocktail), not necessarily because 1 pill is dangerous but because some people are going to take 10. Also some people take pharmaceuticals that either raise or lower potassium levels so what is too small of a dose for some people would be dangerous for others.
The 12,000% of your daily dose for most things is a scam with the possible exception of vitamin B₁₂ for which many people have bio-availability problems.
However, oral potassium does have other effects. The proton pump in the stomach works by exchanging K+ for H+. Excess potassium may affect its functioning. There are several reports of nausea and pain with potassium supplements [1], although I've been adding KCl/NaCl "lite salt" to my work lunches for a long time with no problems.
1: https://jamanetwork.com/journals/jamainternalmedicine/fullar...
For what it's worth: I've used "losalt" drinks for potassium-replenishment (especially during carbless diets), and it works well. Hard to overdose, because it will induce vomiting if you get reckless with the ratio of water to losalt.
You can get sufficient potassium by eating 2-3 meals with plenty of vegetables or leafy greens every day, and you can get sufficient iron by eating animal meat (unless you have a diagnosed iron deficiency and need to supplement with it directly).