As for 1, recall when Anduin marched up to the Paragon of Bastion. He was allowed to get very, very close - much closer than Sylvanas would have - before he struck. He would only be able to pull that trick off once, of course, but that’s still one more time than her.
There’s also the fact that Anduin was being directly controlled by the Jailer, rather than simply being swayed to his side. That ultimately made him more dependable than Sylvanas.
As to why Baine was rejected… well, he’s Baine. His main hobby is being held hostage. Not a good look for a lieutenant.
More seriously, Arthas was a Paladin, and Arthas’s generation of DKs were all also Paladins. I’m wondering if there’s something to that. Maybe they just enjoy the irony, but maybe there’s some kind of actual benefit to subverting an existing affinity for the Light.
If I wanted to by cynical - the Light has been framed as a rigid, dogmatic way of thinking. There’s no doubt, no questioning, just total adherence to the cause. It’s not hard to see how a mind like that would be easier to control - all you have to do is swap the old cause out for a new one and boom, perfect evil servant.