About the land being too developed for 415, they could have put it underground to protect developed areas. Isn't this what QC did with A-10 in Montreal when they removed the elevated part of the Autoroute in the harborfront region?
Nope. Anyone traveling from A-10 to former A-720 must travel through traffic lights on surface streets.
How was that ever allowed to be a part of any A-road in Quebec?
In province of Quebec, autoroute numbers are given on a base of future completions. And as of the projects are not coming to any kind of completion, the MTQ sometimes removes, or modifies, autoroute numbering from some segments of roads. For example, the actual A-955, in Centre-du-Quebec region, was supposed to be a part of A-55, from A-20 to Richmond, and the actual A-55, from Richmond north to Drummondville to be A-51. Then they decided not to complete the initial A-55 project, so the actual situation. But some years ago, I don't know why, the MTQ decommissionned the last southern kilometer of the A-955, so instead of terminating at main street of St-Albert, it now ends at QC-122, the last km becoming a rural road.
About the A-720, things are a bit different. As a part of the Turcot Interchange rebuilding, the MTQ made a deal with environmentalists by downsizing the A-720 to a "provincial road" (QC-136) by making the lanes about one foot narrower.
It's the same about exit numbering. For example, the A-50, in Gatineau, starts with exit 134 because the original A-50 project was supposed to begin near Pembroke, ON, and follow grossly the QC-148. It looks like this part of the highway will not be built in a foreseeable future, so maybe, one day or another, the MTQ will change all the exit numbers, with a starting point in the city of Gatineau.
All of this to say that the highway numbering system, here in province of Quebec, may be one of the most structured systems, but it shows lots and lots of exceptions.