Some more legal opinion: https://waitingfortax.com/2016/10/16/brexit-the-important-role-of-the-court-of-justice/
Based on this, it seems that the question of the reversibility (or not) of Article 50 has not been established — presumably because nobody assumed it would ever be triggered. It may go to the European Court of Justice at some point for a ruling.
As for stopping the triggering of Article 50: I simply do not see any realistic prospect of that happening. The issue is too politically explosive. In addition, even as a staunch Remainer, it would be hard to justify. If parliament did block the triggering of Article 50, we’d have a democratic crisis that would play into the hands of the Brexiteers.
This is now a long game. The fact is: Brexit is a mess, there are splits running through the Brexiters, the government has no coherent plan, and there is no chance of exiting the EU in a way that is going to satisfy the Brexiters (whatever the negotiating path and likely form of exit, a large chunk of Brexiters are going to be unhappy). For Remainers, therefore, it is a question of keeping firm on the more solid and coherent position of remaining members of the EU (or, at worst, retaining single market access), making the case repeatedly and clearly, and staying in the game while the Brexit position becomes ever more divided and messy. Two possible scenarios (both of which Blair pointed out): we look like heading for a ‘hard’ Brexit, which will almost certainly be favoured by a minority even among those who voted Leave, and which will cause economic mayhem; or we head for a ‘soft’ Brexit in which we retain single market access, EU law, and freedom of movement, and according to which we will continue to pay into the EU budget, a position that will infuriate many Brexiters, but leave many others (including Remainers) wondering why that is better than EU membership. The Remain arguments will be important in either scenario. This is going to be about building a strong, non-partisan pro-EU movement. Even if Article 50 is triggered next March, the game is far from over — in fact, it will only have just begun.