You are quite right that the park brake valve is in the sys B return line. It is there to prevent system pressure bleed off when the park brake is set (i.e. long term)
When setting the park brake several things happen.
The left and right pedals must be depressed and the park brake lever pulled.
The partially depressed brake pedals operate pushrods and cables which open both the normal and alternate brake metering valves.
The pulling of the lever operates a switch which activates the park brake valve. It also moves a pawl locking pin which will keep the brakes depressed even when foot pressure is released.
Therefore which ever system is pressurised will apply presure to the brakes as long as that pressure is available.
So it is possible to apply the park brake when only system A is pressurised. When A system pressure is lost the accumulator isolation valve will allow accumulator pressure to feed the brake system via the B system brake lines and bleed off will be prevented by the isolation valve (park brake valve) in the B system return lines.
Hope this helps.