Having actually worked at both Brisbane oil refineries (first BP, and then Ampol, as it was then), pipelines from refineries to terminals (where the petrol/diesel tankers load) indeed can contain different products at different times.
HOWEVER, when a product change is made (say from diesel to ULP), the interface (a known quantity of the mixed product) is sent to a "slops" tank, which is obviously a mix of all sorts of different products. This slop tank is then shipped back to the refinery, and is then re-refined (it just becomes another feedstock, actually). After the interface is removed, the clean product is then sent to its particular product tank.
Temp differences on the product end make for minimal volumetric differences, just BTW. Not worth changing the time of day you buy. The servo tanks are in the ground - they are effectively isolated completely from daily (as in diurnal) temp changes.
As for Avgas, typically, there is a dedicated pipeline to the airport, that is not used for any other product.
Dont know about ethanol mixing, but I do know that the fuel is tested by the refinery by its own NATA registered lab before it hits the pipelines. What then may happen at the terminal (where the tankers load) is up to the terminal operators - they may mix additives at that point. Does anyone work at a terminal who can comment on additive blending?