when to add the curry powder
None of these is categorically true. Adding it early or late will give different flavor, adding it at the very end of cooking will usually leave you with an unpleasant raw spice taste unless it is a roasted variety of curry powder.

It does make a big difference whether the powder is added into oil, water, or an emulsion.

For the most "complete" flavor, add it twice - part goes in the oil when your aromatics are mostly done, before adding bulk liquids; part goes in a few minutes before taking the dish off the heat (optionally bloomed in oil).