Eb is a busy man, I think he tried at least three times to kill Raith, but his magic wouldn't touch him.. If Eb used the Black Staff on him and it still didn't work, time to move on. Besides Eb may have figured out that Margaret's death curse was doing it's job, and Lord Raith was effectively neutralized.
Sure he is, but considering what he did to Casaverde I hardly think he would have trouble with nuking them. Hell, I hardly think he would have trouble with them if he walked in the front door. Also, even more strange is that he didn't realise Lord Raith's weakness to physical objects. Sure, magic slides off him. Harry had no problem hurling an object with magic
at him. Eb could have nuked Raith any time he liked. I think he either lied about the attempts, which is why he's so touchy. He doesn't want Harry to find out whatever grubby deal he has going with them. Or Eb was told specifically not to and there are other protections in place.
Even then, why not hire another assassin who specialises in physical kills rather than magic? Goodman Grey would be a great choice. I am sure there are others. Either Eb never tried or he gave up easily because he was forced to. I don't believe Eb went "Oh well, too hard - guess I'll go back to planting corn". He's not the sort. He's made earthquakes happen and volcanos erupt. Also, it isn't like you need to kill the White King. You could kill the others and imprison the White King.
The question is: Who knows that Eb is Harry's grandfather?
If Langtry does, he's stuck with eliminating Harry legally using the Wardens- difficult without Morgan and with Lucio down.
If he doesn't, I look forward to the scene where he orders Eb and then plays Roshambeau with the one wizard who doesn't care about his wards.
Oh, I think the Merlin knows. He's the Merlin after all. Even if not...he could work it out eventually. Not to mention, Eb does care about the Merlin's wards. He is acknowledged to have the best ones. Even Eb admits they are good. I think that Eb would have to bring his A-game for Langtry.