You are mixing up the process of Brexit, with what happens later, they are two different things.
We plan for what kind of result from Brexit we want, and then implement it. Leaving is not negotiable, it is a right. What we do after Article 50 has been invoked is negotiable.
The line that everything has to be top secret so has not to harm future deals is utter claptrap. People we are dealing with need to know our position at the start.
Unfortunately, the Government is prepared to tell Nissan and their like what our future plans are, but they will not tell you or I.
Mrs May, quite irresponsibly, is trying to gag parliament, and keep the public in the dark as long as possible, because she knows that our final brexit is unlikely to please anybody, on either side of the debate. The leavers will not get everything they want, and will be the most vocal about it.