Buko, sorry you all have to go through this--it's so hard to watch one's baby get so upset!
The things that have soothed Cian the most are the loud shh-ing (yes, ala Happiest Baby) and even more so, keeping him swaddled during the diaper change. A swaddle like the velcro-style SwaddleMe things are great for this. With the ones that contain the legs, you can pop just the legs out to do the diaper change. They also make ones that are two-piece, with a sleep sack and a velcro-attached arm swaddle. I have been making a lot of use of just the arm swaddle, without the sleep sack. If he's wearing zip up PJ's, I just snap the top, and keep the PJ zipper only halfway up, so I can unzip it without disturbing his arms. It's really all about keeping their arms from flailing about, which I think scares them and certainly makes them more upset. Cian is much more responsive to shh-ing, etc., when he is swaddled!
Oh, and my 4-year old daughter (who is very into being the helper) loves to put her finger in his mouth to help sooth him. (I make her wash her hands first.) It actually does help, even when done by a 4-year old. But only if he's swaddled. I've also had her hold his arms down and shh him, which is great when I'm changing his outfit.
I hope this is helpful. Good luck soothing her!