I agree that in the end, Corbyn probably wouldn't get even symbolic concessions. But up until then the scenario feels solid, so let's fork it. Corbyn goes to Brussels and gets nothing. Then what?

In the scenario, Corbyn has made a big number about a second referendum, so one way would be to conclude that they have no good choices, and ask the people to rank Brexit without deal, Brexit with May's deal and revocation of article 50. Or if ranking is to hard, ask for which they want, and which they want if they have to choose between the two other choices. Stepping back to let the people decide maybe means a decrease in prestige, but on the other hand Corbyn can then after the referendum get on with his agenda as no result means that the people voted him down.

And in that scenario the Council may not accept a revocation, so then the Court could get to decide whether an A50 notification can be revoked.