IMO, the 16th never repealed/superseded the 4th, 5th nor the 13th, so it’s void on its face (regardless of the language gymnastics on either side).
Because our lawyer/politician class has NEVER done anything that they themselves could not drive a Mac Truck through. IE: “Job security”.
There is NOTHING preventing a tie-to clause so we DON’T have *both*.
Concur on the last point: Start from 0, figure what’s Constitutionally necessary and, IMO, make each State/territory pay their 1/x%.
Nothing except the most fundamental principle of republican government: that no legislature can bind its successors.