Anyone know by direct experience if the CT20b & CT26 actuators are the same or different?

Seen both and looked the same. I don't know if the wastegate spring is internally is the same.

IMO don't waste your money with a second hand wastegate actuator regardless of gen. Buy new kinigawa or mamba or aftermarket where you can add different wastegate springs for different psi and most importantly has a new diaphragm for longevity - for the low cost it's the way to go.

Thanks Tony, the original question still remains unanswered though i.e. is rod length, lever length, rod travel distance, actuation radius etc the same?

The rod length and body remains the same. As for the diaphragm / rate, testing has had then similar - use google / mr2oc.
Beginner enthusiast self proclaimed engineers have measured rod travel under different psi. I have not come across someone who has tested both under the same conditions, so individual tests / measurements is unlikely to be accurate in comparisons.
Hope someone can help you further,