You are incorrect on everything stated here.
For a low IQ person, living in a modern world is very hard.
Similar to an average person doing calculus.
Knowing which plants are good and which are poisonous takes quite a bit of IQ.
You may find a carrot, or you may find something that looks like a carrot that will kill you.
"The Queen has hairy legs" - important information in this case.
The reason the scientific community keeps coming back to IQ is that it is a very good indicator. It is highly correlated to many things we want to test for.
The US military doesn't take anyone below 83 IQ, here is JP explaining why:
RE: What Should We Do With People That Are Not Able To Support Themselves