Been riding for 5 years, motorcycle my only vehicle. Been in this situation once actually, like a week ago. It's hard to tell from the vid, but looks like oh-shit-grab-ALL-the-brakes followed by trying to turn hard while on the brakes, leading to highside.
1) fast or not you can't predict shit like this, a car straight up smashing into the car in front of you in the lane. You could say "muh stay in the right lane at the exact speed limit", but that's bullshit and you know it. Trying to hind-sight a single, exact scenario is stupid as shit. "Based on my knowledge after seeing this video, this particular driver should have been going x miles slower, been watching exactly the right lane, and perfectly braked, knowing what was about to happen, because the video shows what's going to happen, duh". That's 99% of couch-driving advice I see from anyone trying to analyze a crash video *after* they've seen it. Show the first 4 seconds of the video leading up to the crash to someone, without showing anything else, and if the advice doesn't match post-full video watching, you are pulling hindsight bullshit and your advice is bullshit.
That said:
>is it better to engine break
How the fuck do you think engine braking works. You have milliseconds to react, you aren't going to downshift and try to unpredictably engine brake, you are going to "oh-shit instinct" do something, mostly grabbing a hand-full of brake.
>and cut 20% of your speed and then break rather than breaking only?
Reeks of post-couch-analysis bullshit. You don't get to relive the video and analyze, you have, once again, milliseconds to react, and your reaction is mostly automatic without thinking.
Honestly this is one of those "fuck me if it happens". Car comes out of nowhere and causes a crash right in front of you, you don't honestly know whether they will stick to the left or bounce off. Pull brake not panic-hard, go whatever escape path exists, and pray the vehicles don't end up in front of you