Excellent informative post CA , I would go along with almost all of your top tips , except
Spicing I started my Curry Quest using the SnS base and Madras recipes and immediately had excellent results

However I don't bother looking at the recipe now as I have made so many Madras Curries with the recipe I just go onto autopilot ;D but recently ( past 5-6 Curries ) the wife and me have both agreed the Curry was tasting too rich and we have struggled to finish them , yesterday I decided to look into the reason why and for starters rechecked the recipe . I immediately realised I had been trebling up the amount of Spices whereas the recipe calls for 0.5 tsp of ground Cumin / Coriander I have been putting in more like 1.5 tsp of each thinking more is better :

All this does is give the Curry a much stronger deep flavour being over rich for our taste , yesterday I stuck rigidly to the recipe and the Curry was peeeerfect.
Top Tip.
I feel you need to ' balance ' the amount of Spice in the main Curry with the amount of Spice is in your Base Gravy AND follow the recipe :

Spiced Oil.
Not necessary , our favourite T/A doesn't use it and they turn out the best Indian food I've tasted .
Clean Pan .
Top Tip.
I get better results using a clean frying pan for each portion rather than start a fresh Curry with the remnants of the previous one in the pan .