When revealed, each opponent must discard 1 power from a card he or she controls, if able. Then, if this your revealed plot card, trigger the "when revealed" effect on the top River plot card in your used pile.

Can I just check whether there is any timing thing going on when someone plays Crossing the Ruby Ford? ... ie.. is there some sort of moribund state for power

I'm thinking of a scenario where the plot card reveal may cause me to gain power (for some other reason) and also lose power, for CtRF

Lets say I am on 14 power, it is not Winter and my plot card is Winter Festival - so I would claim 1 power for Winter Festival - I lose inititive and CtRF's When Revealed is triggered first

What happens - do I go down to 13 power before I claim the power for Winter Festival?

The other query is whether I can discard the power from someone in moribund - e.g. someone has played Valaar at the same time and that had been triggered first

You can definitely discard from a Moribund character. It's still in play for all intents and purposes. I think the Moribund state is limited to actual cards and not tokens.

"What is "Moribund"?

After a card is forced to leave play (by being killed, discarded, returned to its owner's hand or deck, or moved to its owner's shadows area) that card is considered Moribund for the duration of the action window..."

#Moribund

Power tokens are not moribund. So if CtRF resolves before Winter Festival, you lose the power before claiming another.

Related to Crossing the Ruby Ford, what happens if the opponent has no power to discard when this plot is revealed? Can I still trigger the "when revealed" effect on the top River plot?

thanks

The "if able" means that you can.