You often have the situation in legal science and practice that a certain situation is not clearly addressed by the law. In these cases you are left to interpret the law. That means you infer from the cases that are addressed by the law to those that aren't. There is a whole lot of techniques to do that (by the word if the law, by its context, by its history etc.). There is a lot of arguments you can make (e.g. to consider an extreme case that borders to the absurd, called argumentum ad absurdum). That is what you learn at law school. Everyone can read the law, but to apply it you need a lawyer.
Given the sheer amount of ways to make your argument, it is no wonder that lawyers often disagree drastically in their interpretations of the law. So which opinion will prevail here remains to be seen. My personal 2 cents are that since the European treaties are designed to build a stable and lasting Union, most probably the opinion will win that you can cancel the process. But who knows, I could be wrong.
Let me elaborate more on the possible opinions on this case and where my opinion comes from:
Those opposed to the possibility to cancel could argue that in Article 49 you'll find the rules of admission to the Union and in Article 50 you'll find the rules for leaving the union. So you have clear rules for admission and clear rules for leaving. The absence of rules for cancelling means that this shouldn't be possible. If they would have wanted the possibility, they would have added it.
Those on favour of the option to cancel would say that it was merely forgotten when the treaty was drafted (that happens more often than you think). They would argue that nobody expected that a country would actually use the option (at least not anytime soon), so they wouldn't bother too much to consider all the cases.
Another argument in this direction is the intent of these regulations. The admission process is to make sure that a country is ready to join the Union, so you need a process to make sure of that. The process to exit the Union was installed because it doesn't make much sense to keep a country that doesn't want to be a member, so give them a way out. Now if a country changes their (collective) minds, would it make sense to not let them? The admission process is unnecessary since as a current member they are clearly ready. And since they don't want to be out anymore, the intent of the exit regulations is not met anymore. So why force them to continue?
Now, whatever argument you find more compelling (for me it's the second one), I believe a cancellation would be accepted. Why? The current situation is the Britain wants out and everyone else wants them to stay. If the UK change their minds now, there will be nobody anymore who wants them out. Since you can argue that cancellation is possible, why would anyone take the other side?
And even if there would be countries vetoing the cancellation, the case would probably go to the European Court of Justice in Luxemburg. There the UK would argue against the rest that they can stay (sounds absurd, doesn't it?). The Court has a history that in a case of doubt they would tend to take the decision that makes Europe stronger. Since Europe is stronger with the UK, I think chances are good that they would allow to stop the Article 50 process.