Road bike to CX/Gravel conversion
I'm looking forward to gaining more tyre clearance by swapping the current 700c wheels with 650b or even 26 inch wheel.

Even if you mounted a pair of long reach calipers (probably something like the Tektro 559, which has a reach of 55-73mm, but I'm not sure if they're still made), you would probably not have enough room to fit big tires. The chain stays, seat stays, and fork blades won't have enough clearance for gravel width tires. That's the main determinant of how much tire you can fit. If you have calipers, try measuring how much clearance there is between the stays at the midpoint of the tire, then move the calipers down about 20mm, then see how much additional clearance you have. I suspect you have very little. Also, if you mounted 650B wheels, you would now be riding wheels with a shorter vertical distance than the bike is designed for. You risk hitting pedals on obstacles on the ground.
I don't know where Cinelli says that this frame is capable of gravel, but 28mm tires are fine for many unpaved roads without loose gravel.