Hi Jamie
Most shop bought Indian Pickles are mustard oil based.
So you don't need any extra in a Restaurant Achari as it may overpower the dish.
If you do start a dish with Mustard oil instead of vegetable oil, make sure you heat
it until smoking and it looses it's colour or you'll end up with a bright yellow curry.

Hi Nj
Dried Raw Mango (Amchur) is the souring agent in this dish, it just adds more acidity to the dish, you don't need to use it as you have the pickle, but I like it in my cooking.
If it was a South Indian dish you would use Tamarind as a alternative.
BTW I made a new Dhansak yesterday with a glug of Maggie Tamarina as an extra, with excellent results.

Love the stuff ;D
hope this helps
cheers chewy