I read Parsefone’s reply as saying the same as mine did! The PLP want Corbyn out because of what he stands for, and they’ll use Brexit as a means of splitting off the membership if they have to.
The Labour leave vote isn’t tiny - like the LibDems and SNP it was around 30% of voters from GE2015, but crucially it’s located in Tory/Lab marginals. I’m not too sure that going hard remain would have that big an impact on polling - while I agree that the Labour Leave vote is stickier than it might be with any other leader, I don’t think there’s a huge dividend to be reaped by aping the LibDems.