Two things:
- as you and Seanm have pointed out, Labour's preference does not matter much at the moment as they're not in power. The Conservatives have shown they are good at holding on to power, even in desperate times.
- the small matter of timing. You assume that Labour can seize power and form a government, throw off the veil of cunning tactics and have the democratic policy debate with members it avoided at the last conference, agree on a suitably democratic position of Norway + CU, sell it to Parliament and the rest of the country, get negotiations moving with the EU, maybe repeal Art. 50 while explaining to the EU they still want out ("will of the people, you know"), get some sort of buy-in from Brussels and 27 member states (sufficient to get more extensions), negotiate a few opt-outs Corbyn wants, and all this in time to avoid a March 29 hard exit. Piece of cake.
And you think the other side will go along with all this because they prefer Labour cakeism to Tory cakeism.
All this is possible, but I wouldn't bet the shop on it.