I think there are two separate issues here.
Firstly your drain test is standard practice for checking to see if there is water as discrete drops or larger amounts gathered at the bottom of the fuel system. This mostly arises from condensation in the tank. If there is water then you should drain it out until there is no more when tested.
Secondly - the alcohol tester is to detect batches of motor gasolene that have had alcohol (ethanol) added by the fuel supplier. This is becoming increasingly common, in the UK at least. Wise people state that alcohol in the fuel is not a problem for the engine itself, but that it attacks rubber parts of the fuel system and the tank linings which can lead to debris in the fuel system, blocked jets and filters and so on. I am not an expert, so I am happy for others to tell me I've got this wrong, but its what I think is the current position.
I don't think any of this applies to Avgas.
By the way its not called a "header" tank if its at the bottom of the fuel system. A header tank is at the top, as the name suggests (like the expansion tank in a car's coolant system). But we know what you mean
H