Margarine is disgusting and quite probably pretty unhealthy, nasty stuff. Here in Sweden it was once promoted as a much healthier alternative to butter, poppycock.
I myself live in a family of LCHF:ers and do myself try to not eat too many carbs, but I'm not going to go preaching about it and I sure don't want to debate about it, but living by that philosophy the problem is usually that I can't have my rice or nan's with them, so I guess a really good BIR dish with all the good stuff included isn't really healthy by anyone's diet.
About oil though, I always use canola oil for my curries, which according to many is one of the healthier alternatives of oil, and wiki says this,
Canola oil has been claimed to promote good health due to its very low saturated fat and high monounsaturated fat content, and beneficial omega-3 fatty acid profile. The Canola Council of Canada states that it is completely safe and is the "healthiest" of all commonly used cooking oils.[18] It has well established heart health benefits[19] and is recognized by many health professional organizations including the American Dietetic Association, and American Heart Association, among others
Still, it's oil so if you're concerned about eating too much fat use less.
As for ghee, I think butter along with the canola oil and the olive oil are actually among the better fats you can eat so as long as it's actually real butter ghee I won't stay away from it, but other kinds I would. But to each his own.
So, now to the dish, I find the technique with the onion and the blender very interesting, because I have too been a bit concerned with getting some fried onion taste into the dish without it being visible, both for madras and for the vindaloo (I've never got any pieces of onion in that either, but I still feel it would benefit from the taste). JerryM's technique sounds good but difficult so I might start out trying your idea for my vindaloo to see how it works out, and maybe also your madras some time next week. It looks surprisingly brown but definitely good!