Debt dynamics aren't your strong point.

He inherited debt of 42% of GDP, and left a level of 66%

When you inherit an economy that has had as much debt thrown at it as Canada had,  and the deficits are exploding, just wrestling those deficits from disastrous to merely bad is a major achievement. Our modern understanding of debt vulnerability is that it is the speed of accumulation that is the biggest problem, and PET's runaway deficits of his last 3 years in power put the economy into ER surrounded by crash carts.

Mulroney turned a 2.7% Primary deficit into a 1.8% surplus, and cut PET's 8.1% Deficit into only 4.4% within 5 years - that is hard stuff outside an IMF program, and only slightly behind Irelands adjustment between 2011 and 2013 . 

Could he have got that down to 4 - 3.5%? Possibly, but that degree of adjustment, when unemployment was so high, and real wages falling would have been brutal. He would have been better admitting defeat and restructuring the debt.