Re: EUCJ
One of the roles of the court is to interpret EU law in disputes between the Commission or other EU institutions and member states. It does not "arbitrate". Insofar as the NI protocol kept Northern Ireland in the EU single market, with EU law surrounding that market enforceable within its territory, why would it not be the ECJ which determines whether EU law is being properly interpreted? The UK will have the same standing before the Court as any member state. The purview of the Court is limited to aspects of EU law which remain valid within Northern Ireland.
If you wish to be in the single market, you must obey its rules. One of those rules is that individual member states accepted the authority of the ECJ to interpret EU law where there are disputes over whether it is being properly applied.
In its role as interpreter of EU law, the court has ruled against the EU institutions in favour of member states on many occasions. As indeed the UK Supreme Court has ruled against the UK government when interpreting law. Interestingly there have been no initiatives coming from Brussels to curb its powers as a result such as those currently being floated at Westminster as a result of the rulings of the UK Supreme Court.
If the UK intends to respect the EU law pertaining to the single market, as it agreed, in order to keep Northern Ireland in it, why would it reject the role of the ECJ, which is accepted by every other participant in the single market?