I think you:

a) misunderstand the EU's incentives here - if a genuine supermajority in the UK come back asking to be let in, EU leaders have an incentive to make that happen easily as a signal of the total failure of Brexit and hard-line euroscepticism - the optics of the UK returning would be so good for the EU; and

b) overestimate the 'punitive' measures the EU could take - worst we would get is the sort of arrangement other EU states have such as hypothetical future euro membership that neither the UK nor rest of EU would have any more desire to implement than is the case for (e.g.) Poland or Hungary; they wouldn't even need us to join Schengen because that actually would create the Irish hard border that the EU negotiations were all about avoiding (since Ireland isn't a Schengen state).