Your cakes look flat and sound like the texture and taste were good- congratulations!
re: sides that stick, I would say that this is caused by a combination of lowering the oven temp, using cake strips and not running a spatula around the sides before unmolding the cake from the pan. Also, a 6” cake will naturally have less browned sides than a larger cake because it takes less time for the oven heat to penetrate to the middle of the pan and finish baking the cake. Finally, chocolate cakes stick more than other types. I would suggest pressing a spatula tight against the pan side and running it around the circumference before unmolding. Also, just to check, did you use shortening to grease your pans? Either that or clarified butter, regular butter will allow the cakes to stick more than clarified or shortening.
re: batter that looks curdled, this is a temperature thing, but doesn’t usually produce serious flaws in the cake. I don’t have my RHC book to look it up, but there is an ideal temp for the butter, eggs, water etc. (68F? 70F?) If you let everything sit out longer- eggs, cooling cocoa mixture, butter, that might solve your curdling problem. The fact that it curdles when you add the cocoa mixture makes me think it may be too warm (or too cool if you were speeding cooling in the refrigerator). When I make a cake with cocoa that is bloomed in boiling water, I do that first, then prep pans, etc. while it cools. If you have an instant-read thermometer you can always check to see if it’s ready. When you increase batch size to make larger cakes, this will take longer to cool.
re: Ghiradelli dutched cocoa, I’ve never seen it here, we seem to only get the “natural” (non-dutched) type from Ghiradelli, so I can’t advise you about cocoa butter content. A quick email to the company often brings a reliable answer. Ask for the fat content in 100 grams of the cocoa, because if they give you the amount for a single serving it is often too small a portion to be accurate. But if you’re not having a problem with dry cake, then I wouldn’t worry about it.
Good luck! You’re a good friend to be undertaking such a large project.