It requires compliance with single market regulations in NI, which would not be required in GB
That goes against the GFA according to both DUP and Lord Trimble (who was awarded the Nobel Prize for his efforts in development of the GFA).
One the principles of the GFA is mutual consent. Changes to NI status from the rest of UK is possible only with mutual consent. Some in Ireland may want a border in the Irish sea, but it appears that NI do not.
He can no longer just sit on his hands because the law says that if he has not got a deal by 19th October he must request an A50 extension until 31st January
Any previously passed law can be overturned. A GE would likely result in a coalition between Brexit and Conservative Party. If they had a majority that resembled the number of constituents that voted leave in 2016 (400+) they would be able to get law changed easily.
Recent polls show that more people fear Corbyn as PM more than they fear a no deal brexit. So labour's chance of winning a GE don't look good. Might explain why they are opposed to a GE?
That Johnson has to request an extension to Article 50 does not force the EU to agree. EU have previously said they would not agree to any further extensions unless there were compelling reasons such as a further referendum or a GE. However, both are ruled out.
EU would then face a dilemma.
If they refuse to extend then UK is forced out the EU without a deal. Then all the bleats about UK being difficult, GFA will collapse and no trade deal with USA, etc., go away as no deal would be a result of EU action as opposed to UK not following its own laws.
If EU do extend without the conditions for extension being present (further referendum or GE) are they not advertising that they don't want the UK to leave at all? Article 50 does not state that a member can not leave the EU unless an agreement is made that all other members are happy with. So it would be impossible and illegal under EU law to extend forever with the purpose of keeping members locked into the EU forever. That would be BAD FAITH by the EU.
Another alternative is that Johnson sends a second letter to EU stating that UK does not want an extension and lists the reasons why. That way he has complied with UK law.
Reasons for UK not wanting an extension could include, but not be limited to;
EU has stated WA can't be revisited anyway, so what would an extension achieve?
Article 50 does not require an agreement.
WTO provides a mechanism for EU and UK to continue trade until other arrangements are need.