Valve to tub spout tube is.
Why no pex for tub spout drop.
Pex when you pull up on the diverter the entire spout moves.
So it s fine to have 1 2 pex going to the shower head.
The interior diameter of pex is actually a fraction smaller than copper this forces water to come out of the shower at the same time as the tub spout.
With a built in diverter you could use a pex drop eared ninety degree female adapter and a nipple but that is more trouble than it is worth.
There is no way to secure pex adequately to keep a tub spout from flopping around.
The tub spout can be copper brass or galvanized pipe.
2nd reason pex is flexible and the spout relies on the pipe for support.
Pax has a smaller inside diameter than copper when you will turn on the spout your showerhead will also be dripping if you read the manufacture instructions it will say do not use pex for spout level 2.
There is no stacking issue with the.
You need an unrestricted flow out through the tub spout.
You cannot use pex because the pipe coming out of the valve needs to be able to support the weight of the tub spout and handle the abuse of pulling pushing down of the divirter built into the spout.
Eventually the line reaches the shower head and one gallon of water will exit there for every 4 or 5 that comes out the tub spout.
Long story short it is unacceptable.
Valve drop to spout tube is within the 6 11 inch tolerance.
Copper keeps the spout in place.