It's interesting that the Lord knew his spymaster would've put a stop to the plan. It's either that the Lord thinks the spymaster's caring for the daughter is a weakness, or he knew on some level the plan was bad, but wanted to go with it anyway (which is a very childish thing to do; "I know Mom will tell me 'no,' but I wanna so I won't TELL her.")

I would not sabotage it just to sabotage it; now that you know about it, see if you can turn it to your (and the other PCs') advantage. Depending on how much your PC cares about his Lord, you might even sabotage it to remove this Lord or to gain more influence so he can't actually pull things behind your back anymore, making yourself the power behind his throne or making the heir inherit ... early.

What I would do, OOC, is talk to the other players and see if they want to make the plot about taking over Faircastle, as the GM has pushed, or make it about taking over your own territory from your Lord. Whether the lord of Faircastle should be an ally (to your PCs) or an enemy. One possibility, if Faircastle has a daughter or sister, an exchange of brides could work, and unite the houses as allies. Force your Lord's hand and finagle the power-behind-the-throne angle, there. Get the loyalty of all the hands that actually DO anything to be to your character or the heir, so that if the Lord gives an order, permission to actually execute it is passed by you and the heir. If you don't agree to it, the order is quietly put on a pile of things that never quite get done. "Yes, of course, my Lord!" they say, and then it doesn't happen. Surround him only with your own people, so he can't give an order that will ever be heard if you don't want it to.

I mean, he's clearly an idiot. This is for his own good.