if i (use bran to) reveal feast or famine in the first action window of plot phase, then reveal a new plot in the FW step. Do I get the +6G and the new plot's G?
if i take the 6G i assume i must also accept the 'cannot make challenges this round' part of it as well

Correct, the decision to add 6g to your total and not make challenges is a "lasting effect", which continues until the end of the round.

Once you've made that decision, it applies to the round. Flip a new plot card, get its effects, and in marshalling your gold will be (revealed plot income) + 6.