That's not what Im suggesting. I'm suggesting committing to doing that during the negotiations, not before A50 is triggered. Likewise this isn't about Government being subservient to Parliament. It simply means bringing the debate about the mechanics and the aims of the trade deal into the public forum of Parliament rather than behind closed doors. The decisions stay with Government.
The uncertainty would be temporary, a bad deal for business & industry would be locked in place for a generation. Besides, an open process would mitigate that uncertainty by giving May & co the opportunity to give some limited feedback on the process.
As I say, a commitment to consult through Parliament doesn't mean delaying the timetable, it can be done concurrently. However its also worth pointing out that there's very little prospect of the trade deal being struck within two years. Even "simple" bilateral trade deals take well over a year, and this is a 28 way trade deal with all sorts of messy politics involving the future of the EU and the influence of Trump and Russia running through it. This deal will be comparable to TTIP in complexity, so to imagine that we'll do it in two years is wishful thinking. In reality, we're going to either need a transitional deal that looks very similar to what we have now or the agreement of the EU to pause the two year clock. Either way, we need the EU's support, and if the EU don't want to play ball because they want us out asap, then we're already stuffed.