Quick help.

Can't do it... You need a long cage RD for anything above 32 teeth. Depending on the frame, a long cage might be prone to hitting trail debris. i.e Rocks Logs, ect
For DH I would run a road deraileur like a shimano 105 with a sora cassette.
DH specific rear derailuers are just a ****ing joke, a marketing ploy. I have not came across any rear derailuer that can withstand abuse

Wrong.

I run a single chainring with a 11-34 x.9 medium cage. Works fine. Look up the tooth capacity on the manufacturer's website.