I don't.
> German train crews relentlessly checking and re-checking and re-re-re-checking
That's done every time a crew changes. The same crew does exactly check you once, but they go multiple times through the train.
This has nothing to do with immigration or crime. Train crews have no authority of the police.
> At the first stop inside the German border, police -- that is, officers in uniforms which read POLIZEI -- boarding the train and carrying out a check of every passenger's identification. This was not a ticket check, it was an identification check.
That happens randomly or during special tasks. There are also police checks on the road near borders, where they check some cars they find interesting. Sometimes they also do coordinated searches for drugs, stolen cars, ...
Targets are illegal immigrants and criminal activity.
> the purpose of my visit to Germany and the duration of my stay in Germany. This was asked despite my already having legally entered and having remained continuously within the Schengen area (my passport, already displayed by that point, contained the entry stamp, ironically from an airport in Germany), and despite my already having entered Germany twice on that trip.
These are standard questions of every kind of border control. It's just that in the Schengen area the controls between member states are more or less random and not necessarily at the border.
Some people may never see a border control and others might see it more often. But for somebody from Poland entering Germany, the checks are only to have an eye on criminal activity. Other than that polish persons can freely move and work in Germany according to EU law. A lot of them work here. Legal and illegal. Illegal is work, if the work is not official registered, no taxes are paid, etc.
> As a result I am extremely skeptical of the idea that Germany has an "open border" or "no border controls" for intra-Schengen travel.
You can imagine that criminals are much more clever than you when it comes to avoiding the few random controls... they also have people broadcasting any police activity. A patrol on the train? Leave the train and take a private car over the border using a small street somewhere...