How to remove sour taste from curry

You can't take the yogurt flavor out, what you can try to do is balance the undesirable flavors and smells out. This may or may not be effective but it's worth a try. You can balance the sourness with sweet and/or bitter flavors. You've added some demerara sugar which is a good idea, for bitterness you could add fenugreek if available. There are comparatively few bitter spices in most people's kitchens, but you will likely have instant coffee and unsweetened cocoa powder, and you may have bitters if you make cocktails. I would shy away from these though as the flavors they would add aren't compatible but YMMV.

If you don't have fenugreek (or you have added some bit it still isn't enough) you could try to add more curry spices like cumin, coriander, and mustard and try to overpower the yogurt taste. How I would do this is to put a small amount of oil in a pan and heat, then cook the spices in it for a short time, say one minute. Then add a couple of ladles of your cooking liquid, and simmer it for a 3-4 minutes before adding it to the dish. This will get the flavors out of the powder and help get them into the dish, if you just add powdered spices to liquid you can sometimes get a powdery taste from them, especially when using a slow cooker.

This may end up making your curry way too strong, even if the flavors are in balances, if that happens remove some of the liquid and replace it with weak stock or water.

Growing up, my mom (Italian-American) would make curry using a bechamel-like white sauce plus curry powder. She said it was her mother's recipe, and I can only assume it was attempting to recreate a meal that she had using ingredients and techniques that she knew.

There are is an advantage of using a white sauce over using just cream to enrich the sauce -- the starch will keep the sauce from breaking over higher heat, but it can get a bit too thick and keep the other flavors from really coming through if you make it too thick, and I don't know if the flavors meld the same way as with coconut milk. (it's been years since I've had this dish, and I currently avoid dairy)

I've never tried mixing bechamel to something that already had coconut milk ... but I suspect that it could work. I know that I've made a white sauce using vegetable molks before (almond & soy ... coconut is the wrong consistency on its own, but there's 'coconut milk drink' that might work, or you could try thinning it with water)