Some example cross sections of what Adelaide Road could look like. These are for a future 26m reserve. Naturally the two-way traffic could be two-lanes one-way, paired with two-lanes on King Street (as per the N2A option). (Naturally two way traffic is better from an urbanist POV though...)

A 21.5m version of these is obvious, just narrow the two traffic lanes to one (wide) traffic lane, paired with King Street. As we know a few chunks of the reserve are already widened to 26m, and much of the rest is currently occupied by empty space, or small twilight buildings due for replacement anyway. The heritage buildings at the John St intersection could easily be moved back 4.5 meters. So widening the rest of the road reserve shouldn't be a problem (except for cost naturally).