I like Trudeau and the Liberals, and I am also a federalist. But 50%+1 is a better policy than "clear majority - undefined." Mulcair is correct that the latter option makes it easier for people to vote to separate and not actual fear separation will happen. Look at 1995, where the question was so soft and vague that millions voted in favour of it, most thinking that it meant there would only be renegotiation of confederation with Quebec. But behind the scenes the PQ was preparing to unilateral declare independence. So you need clear questions, but you also need a clear victory condition. I don't agree with 50%+1 on a single vote (I think it should be 2 votes, 3 years apart, both need 50%+1 to pass), but it was what the UK chose for Scotland.
You need a clear question and a clear victory condition. Imagine the chaos if there were a referendum today and 55% voted to separate. What would Trudeau do? A majority want to leave, so shouldn't they have the right to self determination? What if it it was 60%, or 65%? We need clear rules. We don't need it at the moment because separatism is at a low, but it could come back in 10 or 20 years. It's easy to disagree with Mulcair, but he at least is taking a stance, where Trudeau has never taken any on what would result in a victory for separation.