Summing up my thoughts:
1) There's much talk about the "missing 5%". There are two senses in which this might be interpreted. First, that something is being purposely withheld from us. Second, that some restaurants use ingredients we may be unaware of. I'm not convinced of the first, and agree with the second. BIR style curries have been discussed among enthusiasts for years, with input from ex BIR chefs, behind the scenes footage, and so on. It seems highly implausible that "the ones in on it" are averting our eyes from "the secret". Plausibility, not possibility, is interesting here. Ingredients, methods, and convictions vary from restaurant to restaurant, still more from region to region, so the suggestion borders on conspiratorial. To the extent it's claimed that the BIR taste is elusive, I just take that to mean a restaurant is using other ingredients or the same ingredients in different proportions or brands. However, I don't take there to be a specific ingredient/set of ingredients known by all good BIR chefs but withheld from us. Any talk of being able to reproduce the taste in a restaurant but not a home setting seems to me an artifact of the mind.
2) I've tried many base gravy recipes. The last base I made contained relatively little oil (120 mL for 2 kg). It was also the first time I made my base using the "add everything to the pot and boil" method. Every other base involved frying onions, garlic and ginger paste, spices, and blended plum tomatoes (or some combination of these). I'm not convinced that frying anything separately makes a difference. Since the non-frying method is easier, healthier, and just as tasty, the amount of oil typically used is superfluous. Re-using oil increases trans fat content, which makes it even more unhealthy.
3) I've tried many pre-cooked chicken recipes. My usual method is this: boil a chicken breast cubed in 500 mL water, one stock cube, and one teaspoon of mix powder. I don't understand the complaint of tough chicken. People warn to cook chicken for 10-15 minutes at most and even to run it under cold water to halt the cooking process. I've never done this. I boil chicken on low heat for 25-30 mins and keep it in the hot water, sometimes for over an hour. It's always tender. I tried Chef Din's pre-cooked chicken recipe. This involves cooking chicken in its own juices without any additional water. The idea is to stir for 10-15 minutes until the juices are released, cook, then let rest. I didn't notice a significant difference, and certainly not one that justifies the additional effort. My brother thought the same.
4) Chef Din's comment about sugar is... something. Someone said earlier in the thread that "wannabes" mention the Maillard reaction to create a pretense of knowledge. That might be true, but it doesn't change that the Maillard reaction actually occurs. In my experience, adding base gravy incrementally actually makes for better curries.
5) On the topic of things that actually work, I notice a difference when using a small amount of very finely chopped onions at the start of savory curries. Before trying this, I used a caramelized onion paste (see: "How to make Onion Paste (a Bunjarra for extra flavour in Curries) by Misty Ricardo.) Chopped onion works just as well, is easier, and more healthy, so I'll continue using that.
All in all, I don't think BIR curry making is as complex as it's made out to be, and that declarations to the contrary may be a case of the sunk-cost fallacy.