No, the numbering isn't attached to the Quebec system, they were using two digit Autoroute numbers on freeways that barely cross a metro area (the numbers should be reserved for long distances between communities) and there would be no numbers left for the rest of Canada arranged from 1 on the west coast to 97 on the east coast (99 reserved for Newfoundland and Labrador connection).
Yes, US numbers continue for North-South highways with the ultimate goal being to have continuous numbering on continuous journeys. When you reach the border, you are likely not done your journey so assigning a new route number doesn't add a significant benefit (other than to fix the way the US spread out their odd numbers). US is South of Canada so east-west highways meeting our border couldn't continue without breaking the goal on numbering South to North or using a "0" to denote the most important east west connectors.
I toyed with 5 Vancouver, 15 Calgary, 25 Regina, 35 Winnipeg, 45 Thunder Bay, 55 Sault Ste Marie, 65 Toronto, 75 Montreal, 85 Quebec City, 95 Fredericton to spread the numbers out better, but in a pros and cons analysis realized the numbering could work as it was and choosing different numbers would put aesthetic considerations higher than the journey. When someone reaches the border on I-29 they are most likely going to continue straight on their journey so 29 makes sense from a continuity and ease of guidance perspective.