1/ Let's have a look at this. There's some merit to the political arguments, but the EU legal arguments are weak or confused. It's possible to discuss this without the objective of "attacking" - or defending- Jeremy Corbyn.
2/ The core of the EU legal argument is confused, because the EU legislation on allocation of MEPs provides for the election of UK MEPs, as a Plan B, if the UK is an member at the time of the EP elections. No need for Treaty amendments or EU legislation on election to change. 
3/ The claim that extension of EU membership would be legally "questionable" is unsubstantiated. The possibility of an extension is explicitly provided for in Article 50, without any conditions attached besides a unanimous vote. The CJEU mentioned extension in a recent judgment.
4/ I agree with political assessment that it's hard a) to get the numbers in the Commons for various options, and b) for Parliament to wrest control of the legislative agenda from the executive. But many politicians - including Corbyn - have argued on principle despite the odds.
5/ If you support Lexit, or you think election of a Labour government is the most important objective, fair enough. But don't spout BS about EU law to that end. The last thing we need is some kind of left ERG. //
