following the most recent faw and restrictions i'm looking into a martell summer deck, with lords and ghaston grey. my question is, is scheming on the sands effective or one of those cards that just doesn't seem to be useful? any thoughts or comment swould be very helpful, thanks

The question to ask isn't whether or not the event is playable. The question to ask is "what additional plot is so important that it is worth the investment of 5 influence or 2 nobles?" If you have a good answer to that question, the event is playable. Otherwise, it probably isn't.

ok i guess i have just one question, after plots are revelaed, when is the next player action?

After all plot effects are resolved, plus passives and responses that are activated as a result of revealing/resolving those plots.

Most people will use something like this before plots are revealed, then reveal a new one along with everyone else. That way, they don't have to survive their opponent revealing/resolving a plot that makes it harder for them to play the event.

ok thats what i figured, my question was leading to if i could switch plot to say power of blood after i've seen that they played valar, i know that after valar has been resolved only save/ cancel effects can keep characters from dying and since scheming is a standard player action not a response i wouldn't have an opportunity to trigger it until after all the characters are morbid

Correct. You cannot use plot manipulation to protect yourself from opponents' plot effects. The timing doesn't work out.