> How hard is it to write a law such as "If the same (public) commercial model plane crashes with fatalities more than once in an 18 month period, that model plane shall be grounded indefinitely until a root cause is established and mitigated"
The problem is if you write a law like that, then even if the FAA still otherwise nominally retains its existing power, the mere fact of the "2 in 18-months" rule in statute will have an effect of discouraging groundings with less than two crashes, or with 2 crashes more than 18-months apart. So, unless Congress is going to replace the FAA's decision making role and manage the details of response to each incident, which I think we can all agree would be a disaster itself, its probably not a good idea.
EDIT: Which is why Congress' role of executive oversight and accountability is as important as its role of writing legislation; sometimes there is no good safeguard available through more statutory rules, the best you can do is assure competence and dedication in the executive and apply consequences if there are shortfalls.