A misconception is that the Act passed by Parliament which says we leave on the 29th of March is Primary legislation which takes priority in our Parliamentary system over any motions, which there have been numerous. If (in the unlikely event) May's WA abomination is passed then this will be registered as an International Treaty which we must abide by however damaging the terms are to the UK. If it is not passed then we leave on WTO terms, with no deal, on the 29th of March.
To stop either of these scenarios, would require this act to be amended.
Extending Article 50 after the 2nd July 2019 is not possible as things stand as we cannot be an EU member without MEPs in their Parliament. Now after the 29th March when we leave our seats no longer exist and have been reallocated to other member EU countries in their forthcoming election. The reality is that they cannot extend Article 50 beyond 2-3 months even if ALL other EU countries agree, which is in considerable doubt as several countries just want to see us gone, good riddance, don't let the door hit our butts on the way out, where we are seen as the troublemaker country holding back the creation of the EUSSR.
BTW once Treason May made David Davis', Steve Baker's, Boris Johnston's positions untenable so they resigned and May & Olly Robins took personal charge, there have been no meaningful negotiations where the EU negotiators, May & Robbins are all on the same side (along with Blair, Mandelson, Major, Clark etc, etc) with the same aim to make leaving much worse, so we rejoin in months with no opt-outs and bigger financial payment. May hasn't been clever, cunning or smart where she is as thick as pig sh*t & compliant and easily lead and used as a yes person. If you are looking for a classic case of what treason looks like by colluding with the other side, this is it.
When this is over their needs to be a Royal Commission appointed to look into what has happened and if there have been crimes committed, people need to go to jail for selling this country's interests out to an adversity.
Personally, I think May, Robbins, Blair, Mandelson, Major Clark etc, etc have overplayed their hands and given too much away to the EU which has made the deal unacceptable to the majority of Brexit MPs and the opposition doing what oppositions do (their job is in their title) which is to make life as difficult for Governments by opposing what they are trying to achieve.
Without any majority to amend our leaving legislation, we leave with no deal on the 29th.