Not sure what he/she meant by PST.
Perhaps he/she wanted to tell others the time zone or region he/she is located in.

The obvious reason is to get the material for crafting hellring.
There is guaranteed drop of hellring material at MP10.

Loot is always individual. Hellring materials are account bound.

If you need a hellring, then yes, hunting key is the only route for infernal machine.
By the way, make sure you've got the plan too (from keywarden at A4).