What will the best way for me to travel with a single TfL Zone 2-6 Oyster fare from Croydon to Harold Wood on that day?
Unfortunately that isn’t possible. However, if you do travel via London Bridge and New Cross Gate and do not touch out at London Bridge then you’ll be charged the slightly more expensive NR Zone 2-6 fare.
has this issue been discussed in the London Assembly?
I don’t know. You could always write to your assembly member and ask them.