Is it safe and legal to sleep in car -*not* RV- in Canada (Alberta and BC)?

There's a whole community of people who do this literally everyday. A lot of them blog about it. The most common hashtag is #VanLife.

Sanctuaries

I myself make a point to do this at rural freeway rest areas. That is the purpose of a rest area, and there will be 30 truck spots almost full with truckers doing the same thing and a fair number of cars too. Due to my own "needs" I prefer those with bathrooms, water and WiFi.

Another great place is rural travel centers - Flying J, Petro, TA, Pilot, Love's, etc. etc. These are truck stops who intend for truckers and RVers to park and sleep (and buy). Many have a presentable front area specifically for automobiles, which isn't really to sleep in, but park toward the back and no one will care.

I've camped ~50 days using above methods, and never gotten a tap on the window by anyone.

Wal-Mart doesn't officially do this, but many do "with a nudge and a wink", since the sleeper who comes in to use the bathroom is probably going to stock up on provisions also. However not every Wal-mart is able, due to regulations in the local mall they are in, or local police activity. A community of other stealth campers (or better, non-stealth campers) is a good sign. This is a cultural thing, at Walmart specifically, due to their broad presence in rural areas. Don't think all big-box stores work this way.

Urban vs Rural

Generally you want to avoid sleeping conspicuously in urban areas for several reasons. Local hoodlums operating on foot have easy access to you. Citizens and police will treat you as local homeless living in their car rather than travelers, and you'll be regularly interrupted by police, often with a "can't park here, move on".

In long-distance rural areas, very different. Police want to make it through their day without having to clean up another accident site due to drowsy driving. They see you sleeping, they think "Thank God, a smart one" and roll on by. This will vary, based on locality - Massachusetts not so much - but they should certainly understand in AB and BC.

Your problem will be park rules. National Parks are not keen on casual roadside sleeping, they want you to go to a campground.

And finding parking in places like Banff or Jasper is a challenge unto itself in summer.

You are probably better off tasting homeless living in cities oitside the Rockies.