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.
Why no pex for tub spout drop.
There is no way to secure pex adequately to keep a tub spout from flopping around.
2nd reason pex is flexible and the spout relies on the pipe for support.
Pex when you pull up on the diverter the entire spout moves.
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.
Valve drop to spout tube is within the 6 11 inch tolerance.
Any restrictions cause the water to gently back up as it waits its turn to exit the tub spout.
You need an unrestricted flow out through the tub 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.
So it s fine to have 1 2 pex going to the shower head.
There is no stacking issue with the.
Valve to tub spout tube is.
If the valve has a built in diverter three handle valve this would not be an issue.
Shark bite copper shark bite 90 iron pipe pex everywhere else.
Copper keeps the spout in place.
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.