// Article 50 of the Geneva convention ... //
No it doesn't: wrong Convention, it's Vienna not Geneva, and Wrong Article. Presumably you mean Articles 60-62, but:
Article 62 requires a "fundamental change of circumstances" which was "not foreseen", so isn't relevant here, as the Treaty we're planning to break was signed with the knowledge that negotiations may not conclude by the end of this year -- and, indeed, with the express purpose of providing a contingency for this case;
Article 60 requires a "material breach", ie a deliberate violation of some sort. I am not sure that anyone is accusing the EU of a material breach. They are just sticking to their red lines, as are we, and we knew that this negotiation would be difficult.
Put simply, the Government is acting in bad faith in attempting to pass this law, and to confess to breaking international law is an astonishing point. Or are we supposed to understand that we can pick and choose which laws to obey?
Not to mention, of course, the damage to the future reputation of the UK. What is the point, other countries will be entitled to wonder, of entering an agreement with the UK if they will just break it whenever it suits them?