Looks like a misfire to me.... It will spike lean with each misfire since raw oxygen (and fuel) will be going into the exhaust. That'd explain the lack of power, blinking CEL, and misfire counts.
No misfire codes present at all? That's unusual given the fact that the CEL was blinking, unless it went away for awhile during driving. If it starts blinking again, pull the codes. I bet you'll see a misfire code setting. Which sensor is reading this? It'll help isolate it to a particular bank.
Going to 20% is normal during decel when coasting in gear since you'll be in a nonfueling state. Essentially, you are reading atmospheric oxygen content (20.8%) at that time. Many systems self calibrate during that driving state to establish a free air reference. However, if the OP was under load when this was occurring it is almost definitely a misfire.
Agreed. It's also possible that the fuel cut off (FCO) state that's present during decel is also commanded during a detected shift. I know the LS1 stuff from 10-15 years back had it. Can't imagine why it'd be a whole lot different these days...even across platforms. So, the AFR spikes might be normal and a separate issue from the misfires.