Yes, that sentence sounds correct and natural! It clearly conveys the idea that you plan to share information with others, even though they wouldn't know it otherwise. If you want to emphasize your intent further, you could consider slight variations, but the original is perfectly fine as it is.

It's not technically wrong grammar, but the logic is dodgy. Don't simply say they won't know, because they will know (when you tell them). Better to be more specific - for example: "Nobody will know, until I tell them."

Yes it’s fine. Maybe use ‘so’ instead of ‘but’?