Here's how I comprehend Corbyn's stance towards the EU and the Remain/Leave campaign through the years...
1. General Ideals: Against EU. An imperialistic, federalist, US-aspiring superstate, with a democratic deficit that doesn't care enough about the little people. Doesn't allow state aid or worker subsidies.
2. EU Ref: Cautiously pro-Remain for three reasons. One, his voter base is mostly pro-Remain. Two, his mission is to rebuild the welfare state destroyed by the Tories. Something which years of Brexit wrangling would distract from. Brexit might be the right cause, but the fight comes at the wrong time. Three, with the Tories in charge of Brexit, they are bound to screw the small people over even more. Those reasons trump his anti-EU beliefs.
3. WAB: Don't like it too much, but don't despise it too much either. Realises the real prise is the negotiations that follow Brexit. Voting it through would probably end the stalemate and end the nation's current misery, but he can't vote for it. That would be allowing the Tories to get scot-free with Brexit, dictate the next phase of EU negotiations and continue till 2022. Might even win the next GE. If it ends up a disaster, he could also share in the blame. Whereas keeping the pressure on the Tories could see them collapse and go to a GE sooner. Vote it down.
4. Current intention and People's Vote: Genuinely ambivalent. Leave with a deal would respect the first Ref, would perhaps heal the country and it also coincides with his ideals. Whereas Remain leaves open wounds for half the populace that could fester. However. Leaving even with a deal means a lot of the focus and energy of his Premiership would be on negotiating the future relationship with the EU instead of the social revolution. And that could also put a strain on his party like it did with the Tories and it would distract him from his most important work. Shit, this is a fecking mess. Sit on the fence, pledge to do whatever the people say.
That's the vibes I'm getting. Complete conjecture of course.