(a). any kind of plastic will burn (given sufficient conditions -
the circumstances which create those conditions are highly
unpredictable). why chance it ? use metal ducting.

Even concrete will burn under the right conditions. The chances of PVC
burning are slim.

(b). wouldn't you want hot dryer exhaust gases to flow upwards out
of the duct, vs. down a pipe ?

You don't want any moisture running back into the house or collecting in the
pipe.