If the evidence would be so clear, there would be official guidelines. In reality, such guidelines do not exist.
If the evidence would be so clear, dentists would urge all of their patients to have their amalgam fillings removed. That would be both a great service to their clients’ health, and it would be a good business opportunity. In reality, no dentist organisation, no independent scientific organisation, no health official, and not even an independent university group advises to have your amalgam fillings removed. Only a minority of dentists offer this unneccessary treatment. This shows that it’s only a good business opportunity, it’s a good way to talk innocent people into sending their money into your bank account.
Note that for many reasons, the amount of dental fillings has greatly decreased during the past 30 years or so (at least in my country and the surrounding countries – I’m assuming the same development occurred within the USA). There are much less filled teeth than 30 years ago. Of those teeth that have a filling, a much lesser percentage than ever before has an amalgam filling.
This means that the number of amalgam fillings has decreased by a very large amount, because the above two effects combine. Still, the number of autistic children is rising, as is the severity of their disorder.
This shows that whatever the cause for the rising prevalence of autism may be, it has nothing to do with amalgam fillings.