I double-checked the valve sealing again and found that one of the exhaust valve's is leaking out the carb cleaner being shot into the ports.

Would anybody condone replacing just the one bad valve? Or, should I replace them all as a set?

 

You can do just one. Thats what most shops will do. THey will however do a complete valve job (regrind Valves and seats), if there is any signs of needing done.