Is 72h free Visa valid on Multicity booking?

Yes, multi-city ticket is perfectly eligible and in your case you are eligible for 144 hour visa free transit in wider Beijing area. Also, this is not a visa, but visa free transit and 144 hours has replaced 72 hours in Beijing years ago.

So, it seems that you will just have a short layover in Beijing on your itinerary toward Japan and here you will be covered by 24 hour visa free transit. During your return, you will stay for more than 24 hours, so you will be covered by 144 hour visa free transit. If you have mentioned all stops within your itinerary, you are perfectly good to go!