He'd have to be pretty stupid not to realize that, and whatever else he is Ishy's not stupid. He's the only Forsaken who's ever figured out the DO's end game, so early it's why he turned.

Could he conceal it? The DO is very knowledgeable about the world even when Forsaken aren't involved, but when they are he knows just about everything (probably because of the black chords, or rather the link they represent.) It's uncertain whether he knows but very plausible.

Because they knew Semi was free, that the DB was captured, that Semi fantasized about using it on LTT and that he would have no other options if she did. That's the answer to Semi's pathetic wails asking why she was betrayed: Because she was never more than a valuable piece to be sacrificed at need, and even the queen is worth sacrificing for checkmate. She makes the mistake so many Forsaken do: She trusts the DO too much (i.e. at all.)

Why not capture him and turn him with a circle of Myrddraal since that may be the only way you can prevent someone returning to the Light if they want to enough? If that's not an option the next choice is a chain voluntarily accepted rather than one forced upon him. People usually don't resist fates they choose as much as those thrust upon them.

It's not as coincidental as it sounds because all the pieces of Semi's original plan to collar Rand were still in place. All that changed was that Semi and the DB were both in Rands possession, but once the DO changes that he can go back to his original plan, and that's probably just what he did.