For cards which copy a "when revealed" ability of a plot, eg river / city plots, good for the gander, archmaester Marwyn, etc etc - I presume the "when revealed" part of the ability is until the full stop? Which means that copying things like courting favours, summoning season and building season are essentially uesless? However if I copy feast or famine i would get the 5 gold without any downside. Are I copy in my intepretation?

You are completely off: the "when revealed" ability runs to the paragraph break, so it includes all the text in each example you have given.

Thanks I was a bit confused by the river and city plots but I just realised that have a condition to the second part ( "If this is your revealed plot card…")

Correct. Good for the Ganer would still copy the "then, if this is your revealed plot card" part of the effect - and not meet the condition.