Let me know if anyone has any thoughts on any of these
Yes, I'd like to know:
a) why you have constrained the test to only 5 recipes (there are over 15 tikka/tandoori recipes that I've counted in this recipe section)?
b) how you selected the 5 (of the 15 or more available recipes)?
I think that the following should also be tested for it to be a fair test of available recipes (plus the others I've not mentioned):
- Panpot's (allegedly from a true BIR)
- Infindforu's (allegedly from a true BIR)
- 976bar's
- Layne's (a trained chef, as I understand it?)
- Pete's/Haldi's (also from a BIR chef)
- Tempest's
Are we okay to substitute the tandoori masala mix for tandoori masala paste or powder?
Absolutely not since you are fundamentally changing the recipe!
All testers should anyway state
ANY AND ALL deviations (and assumptions) from the specified recipes. I'd be gobsmacked if many would follow them absolutely to the letter

It would be very expensive to make for someone with no spices in the cupboard
I hope this is not your (and other's) definition of "cheapness" Chris?
It is far less expensive than buying a jar of paste that you don't happen to have on the shelf! The "cost" should simply be the cost of the ingredients, once purchased and available.