So, there's some interesting stuff that happens with Charming and Open, especially between PCs. For that, the choice being there means that if you do lie, the other player knows it, right? Because you don't get to ask. It's not a trade. The Bard asks, you lie, that's the end of it. Well, not always. Because sometimes lying to someone who trusts you and is being frank and open and earnest means you're Defy Danger - the Danger being "I lose this person's trust". Something I've seen in play before.
As a GM, answering for NPCs, you can always lie, but it works the same way. Often, I'll say "she tells you that, no, she hasn't seen your brother, but you can see the way she grits her teeth and won't meet your eye - she's lying to you, she has seen him, but for some reason won't admit it. I wonder why?" and then you make your move, you snowball and push. Use the lie to create additional drama. Remember, there's no risk here, for the Bard, mechanically, it's just a part of the way Bards communicate, so you don't have to give them anything. it's a trade if you do, and often that's really fun, but keep it part of an ongoing story.
The questions are character-to-character and player-to-player both in that by the end, they're things that get learned, but they don't have to be literally "WHO DO YOU WORK FOR?!" they can be surreptitious or sneaky, couched in hidden meaning, etc. Always ask questions; "how do you learn that? Is it spoken or unspoken? What gives him away?" and let the players guide the NPCs a little.
As for asking back, the NPCs are your channel into the fictional world, so think both about what you want to know, as the GM and about what they want to know as the NPC, but keep it in-fiction if you can. Think about what the NPC might learn from the tenor and tone of the conversation. If it's a sultry conversation by candlelight, "whom do you serve" is going to be harder to learn than "what do you most desire".