It's possible to train it, but tricky. I believe de-escalation training would actually work in a seminar where most participants are actually strangers, or close to it. I'll use a non-self defense example.
I am a certified crisis intervention specialists. I don't work in Philly but have a friend who was one of their negotiators and was on the Philly Joint Terrorism task force. He asked me to be a role player, not only because of my training but we both used to share a hobby of amature Theater Acting. For each group my scenario was to be a man prepared to jump from an office building. I knew that my character's motivation was that I had lost my wife the year before and I just got a call my son had died in a car crash. One negotiator, instead of taking their time and starting from neutral he went straight to "what would family think? What would your wife think? Do you have a kid?" I tried to be nice at first, a good partner, showing it was aggitatin me, that I was getting profoundly upset. He kept pushing, so I jumped. That scenario training may well not have worked the same way as people I work with because they know how I react to emotional stimuli. That would give them an "edge" in reading me that you don't get in a typical self-defense situation. So I think it's doable, if you set up realistic scenarios with strangers as role players who are good at what they do.
I also think people look at de-escalation too narrowly. By this I mean a crime might still occur. Example he maybe on the verge of shooting me but my remaining calm and saying "you want the keys to my car? That's fine, here you go" is de-escalation, I didn't get shot. Saying "yo man, I'm sorry I didn't mean to spill your drink, let me buy another round for you and your friends", same thing. In either case I "gave up" something but really in self defense that is what de-escalation is about. You will never get the guy intent on robbery to stop wanting to rob you just with words. You won't get the "beer muscle" barfly who is now embarrassed and needs to regain status among his companions just with words.
The other party is already prepared for violence to GET something. They want your keys, they want to regain face. If they don't get something they will use that violence to get it. The trick is to be able to read what it is they are looking for and give it to them in one way or another. Maybe with the robber you say "hey man I took the train, but here I got some cash in my wallet, or here, it's a [insert top of the line cell phone]. I can cancel my cards, the cash, if I am carrying more than my deductible, is covered by my home owners. The cellphone, they don't have my thumbprint, will never figure out my goofy password and I can turn it into a brick as soon as I get to my computer...and it's insured too through my provider. That, to me, is one of the tricks of de-escalation, understanding that dynamic and that, in the end, what did a really lose? Some replaceable property that's likely ensured? A little bit of pride? I didn't get shot, seems a fair trade.