The new goodman (2 ton,dsz) will not go to cool (reversing valve gets stuck somewhere and the replacement was suggested), but it has no problem doing a proper defrost. Thus I question the decision to replace the valve on the outside unit. Please explain.

If the reversing valve will not activate to put the unit in the cooling mode then the valve coil or valve itself is defective as long as 24 volts is getting to the valve coil during a call for cooling. This all needs to be tested on site.