True. But if I come before 180 days take short stay like this and than departure again for another 150 days, isn't my trip perceived as a longer trip?
Yes, if they stop and look at the whole series of trips ... but they often don't do that at the POE, that's why many people can get away with short visits the US once or twice a year ... until an officer eventually notices the pattern. The most important trip at the POE is the last trip you're returning from, and 6 months raises a big red flag.
On the other hand, I just checked USCIS info and there is: "any lenght of absence may be considered"
That is correct. But the shorter the absence is, the more other evidence they have to find against you.
Also some people told me on another discussion forum that they were also grilled at POE after just couple week vacation.
And I was sort of grilled after a weekend trip. Anybody can be grilled for any random reason ... the difference is whether you can get past the grilling quickly, or get past it at all. Somebody with an 11 month trip is going to have a harder time than somebody with an 11 day trip.
If it is up to CBP officer's decision, than I can be turned away even after 2 day shopping in Canada?
Yes, but not just for a 2 day trip. If the trip is that short, they have to find some other evidence of abandoning residence, like you went on that trip to activate your newly approved Canadian permanent residence and buy a house in Canada and interview for a Canadian job (and even that might not be enough to send you back or take away the card, if the trip is only 2 days and there is no long series of trips before that).
Very right. In this case I though, that I wouldn't be interrogated because I wouldn't stay longer than 180 days for a single trip.
As I said, you can be interrogated for any random reason. The length of the trip is a factor in whether you escape that interrogation cleanly, or get detained or sent back. Your problem is that you don't have a house and spouse and other such ties in the US, so you have to be extra careful with the other aspects of your life and travels in order to have a good chance to get through such an interrogation if you ever run into it.
Being safer would be better to use NYC as a POE and then use domestic flight to Miami.
That looks better on the surface, as long as they don't dig deeper to find whether you actually went to your NY residence and find out that you never went there.