Is 400000PLN per year a very high salary in Poland? If so, what kind of job can make this money?

Again that math is off. How many of those houses in Warsaw are under mortgage? How many of those are parental properties divisible amongst siblings if sold? how many are owned by a single large company or business owner? How many of those houses are co-owned and don't have a single owner? Once you get all the numbers then you'll realize that the reality is very different from imagination.

There seems to be an increasing number of 'syndicates' acquiring and selling shares in houses, and not just in Warsaw or the major cities but even in the back of beyond.

Yes, 10% is too many. It is closer to 1%.