~How would they know where he spends his nights?
This is a very common question for anybody who wants to cheat on the residency requirements and lie on their PR renewal or citizenship applications. They want to know if they will get caught. The answer is that nobody knows. The Canada border works with the US border. If the US border has records of your coming and going, Canada might have them too. I do not know how accessible they are, can they pull it up on their computer or do they have to ask for it specifically? Canada border will of course have their own records of your comings and goings to some point. If these records do not match your application, you would have a problem. Of course they will not tell you beforehand what records they have. Some people lie and get away with it. Some get caught.
~Does a part of a night count as a night just like a part of a day counts as a day?
~And, what if you left at night, say 11 pm on a Wed, and returned to Canada again at night, say 7 pm on Thurs (within a 24 hour period); does that count as a day for PR but not for citizenship, lol??
I believe they mean any part of a 24 hour day so in your case, both Wednesday and Thursday will be counted for PR and Wednesday might count for citizenship if you slept in Canada that night but really, when you would have a really complicated crossing history, it could be anybody's guess how they interpret it. You might get a visa officer who'll give you credit for all your days or one that gives you credit for none.
However, if the point is to be residing in Canada, you really are not residing in Canada if you sleep in the US every night and your apartment or house is located in the US. Still, with the rules for the PR with the any part of a day clause, you would get away with it for PR.