It is OK Cory A, I am not offended, in fact flattered. I think you are absolutely correct with your observations on the ingredients used by BIRs. But it is not purist, there is no simple answer, and technique and equipment play a major part in creating 'that taste'.
I am a sort of BIR with a different approach, but I do take-aways, banquets etc as a professional business; I turned professional about 6 years ago. Our new kitchen was built to support the business, but we made some minor mistakes and one big one - we bought a domestic look-alike range cooker (cheaper) which is almost a complete failure, so we shall replace it with a professional one at some point.
I too have searched for that Holy Grail, and I am happy now with my own selection of bases and the Ultimate of course, as they satisfy the needs of my local market. My customers no longer go to the other takeaways/restaurants in the local towns and villages as the norm, so we must have got something right. They like your good self were our driving force desperate for us to produce that curry house taste before they would make the leap. NO, to be honest we haven't cracked it... yet; we are nearly there and with the new cooker we shall.
We are thinking of opening a full blown Takeaway or maybe even a small restaurant, because we have a lot of backers financial and other wise. I only mention this because I do understand what you are searching for, and yes I have done a lot of observation and talking in the past, and it is different with each BIR. I have even been offered membership of the Guild of Bangladesh Restaurateurs, and may take it to see if it opens the secrets door!
If you wish, I will post on the 'Hints and Tips' what I know to be the standard ingredients, but they will be my list and each BIR will be different. By the way one of the key differentiators, which I have not seen emphasised enough here, is the Gram Masala. This is usually the Chefs signature and what makes one BIR different from another.
As you know the vast majority of restaurants and takeaways are Bangladeshi, and thus the dishes are almost identical in every one of them, so if you crack one you have got the basics for them all. They have their own food associations and journals and share the information we seek between them on the QT of course.
The great variations in dishes arise in the Kashmiri, Nepalese, Indian and Pakistani restaurants, but they sadly are rare, usually very up market or embedded in the appropriate Asian domestic/work communities. This is my area of speciality as I tend to use their techniques ingredients and flavours; that is what differentiates me for the mob here! Mind you we only have one or two decent BIRs on the Island (besides mine of course ;D)
Sorry to be long winded but I thought I should put forward a professional view, and of course I will help in any way I can to find the HG of Birs (if there really is such a thing).
Keep up your excellent work and I am sure you will teach me a few things, I am always learning!! I am certainly going to try the recipes. By the way, my quest is for fast cooked dishes with the taste and I would appreciate any recipes methods that focus on them.
Happy questing
C P