> because it's a secret and you don't want competitors to get advantage
Talk to your customers about their problems and their needs, not the solution itself. The solution can remain a secret.
> because customers won't get the idea without a mvp
Again, figure out the customers problems first.
> because revealing your idea might reduce your impulse
Don't reveal it until it's ready to be revealed.
> because customers will ask for all kind of features and change your idea into something else, more practical, like "get a job" instead of dreaming in your great idea
If customers are asking for features, then there's your demand right there. Strategise on how you want to address it.