I understand Watson was planning to use conference to launch another attack on Corbyn. It could be that Lansman’s motion was a way of putting a shot across his bows and avoiding a more damaging Watson inspired row later on.
As for The Brexit issue, if you want a credible chance to stop it, you need to vote Labour or Lib Dem. Not sure what Watson’s beef is anyway, vote Labour, get referendum, campaign for Remain. Even Corbyn has suggested he wouldn’t campaign for the deal, but even if he does (and someone probably should so that both cases are properly represented) what does it matter, Watson and anyone else can campaign for Remain and you and I get to vote in referendum. Which is what we’ve been after for a few years now.