Corbyn’s support for the Customs Union was in Feb 2018, and the political climate has changed drastically since. The Government has lost two parliamentary votes in 2 days and faces a third, fatal one next week. Labour have smelled blood and have said they will ‘immediately’ table a motion of no confidence against the government. If the Government loses, and May doesn’t resign, and it probably will lose this vote. The problem here is, how can the government negotiate a modified deal whilst it’s in this scenario? It couldn’t. Besides, Labour want an election which makes cooperation very unlikely because if a Brexit deal goes through Parliament, there’s no need for an election. The scenario you’ve laid out is a possibility rather than a probability and I think the die has been cast.
Labour won’t stand on an explicit manifesto pledging to hold a second referendum. Corbyn has actually been deathly quiet on this and his own party criticises him from this. It seems the Labour leadership want to go away and negotiate their own deal and try to get it through Parliament.
The reason Labour hasn’t had a coherent policy on Brexit. Firstly, it’s in opposition, it can afford to be slightly vague. Secondly, and most importantly, it has to appease its traditional working class vote that back Leave, as you shown and it’s metropolitian voters in Greater London and elsewhere. It’s members are also overwhelmingly pro-Remain. Barry Gardiner’s suggestion that Labour would have a referendum on the deal it negotiated is an interesting one, and may become policy. However, he made it clear that’s his own opinion. Labour’s policy will be deliberately as broad and as vague as can be so it doesn’t upset the delicate balence between its voters that Leave and Remain.
As for who would win in an election, it’s guesswork, no matter how educated the guess. Labour are ahead in the polls, but it’s anyone’s game, I do think Labour could win the ‘popular vote’ but the Tories remaining the largest party in Parliament. In 2017, the polls said the Tories should have extended their majority massively but lost it. My guess is that either Labour or Conservative could emerge as the larger party, but, I doubt either will win a parliamentary majority. Labour will have to perform very well in Scotland to stand a chance of winning a majority.