I think many people may think of this the way I used to think.
The whole ‘nn’ Number of carbs per day. I’m a technical guy. I look at things in a way that normal humans don’t. So when I hear ‘n’ per day, to me that means in a 24hr period. PERIOD. So to me that means if for dinner I eat my one meal a day (one meal a day is my prefered way to eat, no longer being hungry all the time
) if I eat that meal at say, 3 pm, and it contains 20 grams of carbs, and I then eat the next meal at 10am, and it contains 20 grams of carbs, then that’s 40 grams in one day. Two calendar days sure, but within one 24 hr period. Unless there is some biological reset that occurs while we sleep 
So I think I can see a little more clearly the gist of your question. Is it a sudden load of carbs that slaps you off the horse like a tree branch. Or is there some kind of biological calculator at work keeping track in some fashion? I beleive the second to be the case.
From what I’ve learned thus far, keeping your insulin low enough so you can continue burning fat for the majority of a 24 hour period is probably what keeps you in ketosis. If (and a big if) you could sip that milk shake all day long, and not go above a certain amount of insulin, then in my opinion, you might be able to overcome the restriction of carb intake. BUT (indeed, a very big but) I do not think it would be an easy feat to pull off. Not only does sipping some carbs increase insulin, proportional to the amount sipped, so in theory you could possibly calculate the amount to sip per minute to keep it low, BUT… there is also an insulin response to the taste. And again, my own opinion, that might be a bigger uncontrollable insulin spike than sipping carbs. It’s a Pavlovian response. Something that we might be able to untrain our bodies from (my opinion again, but if it’s a learned response, we should be able to un-learn it).
Too confusing? I think so. So I started out by just going zero carb. No guessing, no keeping track, just eating to satiety 
Keto Vitae!