Hi Axe,
Essentially, base or gravy, is a stock. I have known it to be called all three. Most bases rely heavily on large quantities of onions along with various spices, pretty much how you describe the stock that you observed.
I am totally with you on how you describe the TA's and BIR's reason's for cooking this way but, maybe the base,gravy or stock, is not as recent an innovation as it may first seem?
If I'm correct, I think the correct term for the base is "Grabbi or Garab" something that was developed long long ago in Indian cuisine, way before it was it was introduced to Britain. Perhaps it was not used in the same way that the BIR's use it today, but if you think of it as just a "stock" then guess, this would make sense?
Lots of cuisines use bases or stocks as their starting point's (as I'm sure you know this anyway

), Italian = tomatoes, french = wine, and so on... I think describing the base as a stock is a very suitable description.
Have you made one yet Axe, if not, what is your perception of how it would be? My perception of what a base should be like, is, a thin, mildy spiced liquid of soup consistency, versatile enough as not to be the dominant flavour in the dish but balanced enough to be the foundation or starting point for a dish, giving you the flexability to build on.
As for a standard spice blend or spice masala, I'm completely with you on this. It's definitely created for convenience. The varying taste of the dish being create by timing, quantity and technique and maybe even temperature?
I think separate spice masala's lend itself more to traditional style's of cooking rather than BIR. The one noticeable thing about most BIR's is that "taste". Each of their dishes will have it, you can't put your finger on what it is, but you certainly know when it's missing.
For example, a traditional Vindaloo will be a completely different dish from a traditional Madras (I know some people won't accept Madras as having traditional origins) Whereby, a BIR vindaloo, albeit, a different dish from Madras, you can tell that they're cousins! Does that make sense

Anyway, that's my slant on things, for what it's worth ;D
Ray