Labour are voting for article 50 after May's speech, once that is invoked two years and we are out.
You are making a whole raft of assumptions there:
1. This timetable is likely to be very dependent on the outcome of the Supreme Court hearing. Firstly on the basis of whether the court rules on the nature of the Parliamentary involvement, and secondly if the Government loses whether they will chose to appeal again.
2. That Labour MPs do vote in the manner you indicate (and that Tories all vote in favour of Article 50) - there will undoubtedly be rebels and on the Labour benches it remains unclear (not really news as it is Corbyn we are dealing with) whether Labour MPs will be whipped or given a free vote.
3. Any Parliamentary process will involve the Lords and then all bets are off.
4. That a deal can be agreed within the 2 year timetable, which looking at negotiating time for other deals of similar complexity (actually most are less complex) seems very, very optimistic.
5. If a deal isn't concluded by 2 years I suspect it will be in everyones interests to extend the negotiating period, which is allowed, which would necessarily delay the actual point at which we leave the EU.
6. That a potential transitional deal could actually involve an extended period as a member of the EU, or perhaps more likely as a member of EEA.
7. That this period doesn't extend to a point when there is a general election and therefore the most recent mandate will be that of the new government and not the referendum vote.
8. That the pressure for a second referendum isn't successful - if there is a second referendum then we have no idea on outcome, nor timetable.
But hey you can chose to live in your naive, simplistic world if you want. I chose to live in the real world.