Why the jump to Mulroney winning in '84? The simple fact of the Liberal-NDP coalition might well kill John Turner's bid. Although Turner was pretty fantastic at gaining control of the Party (one of the Turner books mentions that old classic: sign up 50 new immigrants who can't even speak English to vote for Turner to become leader).
After all he's the most pro-business guy the Liberals can find bidding to lead as PM (however briefly) a Liberal-NDP alliance. The NDP would walk out before the '84 elections. Maybe Jean Chrétien wins, loses the '84 election and then the Liberal-NDP alliance carries the day against NAFTA in '88?
Further simple butterflies perhaps means Turner isn't quite as much of a coward, blows off Trudeau's wanted appointments and doesn't cave in the '84 debates with "I had no choice". Mulroney would still win, I imagine, but even a marginal majority government would have major implications going forward.
In terms of policy, however, Trudeau by this point had already destroyed the Canadian fiscal position[1]; ruined the military; pissed off Quebec, the West, and Red Tory Ontario; and pretty much is in power only because Joe Clark can't count. So most policy is basically dead, except for the NEP.
Will Western rural NDP members (those that would join the Reform Party IOTL) convince Trudeau against the NEP? Will the NDP stop MacEachen's wage controls?
(Note, of course, that neither the NEP nor MacEachen's budgets did much.)
Constitution Act, 1982. What differences might be in it if the NDP is in cabinet?
[1] First he wrecked it. Then he torpedoed Joe Clark's budget when Clark tried to fix it. Then, IOTL '80s he had a series of the worst budgets in Canadian history. Christ I wish Stanfield could have caught one more football.