So I fired off some questions to my Schiphol sources on this one.

For European sectors in Y cabin, it's not possible to reserve a seat before checking in.
So theoretically, the whole cabin should be empty when Internet check-in opens 30 hours before departure.
But this is not the case.
Firstly, pax checking in from destinations further away (say, Singapore for example), get seats before we can check-in as they are able to check-in 30 hours prior to their first flight.
So that takes away some seats.
Secondly, the Departure Control System will distribute passengers in the plane evenly as more people check-in, also taking into consideration the booked load.
With a very light load, certain sections are blocked for Weight and Balance purposes.
So on internet check-in it may appear as something has been "reserved" for us, but that's merely the computer allocating a seat as we check-in.
Of course check-in agents can shift the passengers if they don't like the seat the was assigned by the computer.

Select Seating is no longer able available on intercontinental flights - it has been replaced by Economy Comfort.
On European sectors it is available - for those in Unrestricted Economy fare classes and Silver Elite and upward, provided the FB number is in the reservation.

Does the DCS automatically preseat those who are eligible before check-in opens?
No it does not.
Does it automatically allocate seats upon checkin in, in the Select rows if one is eligible?
Yes it does.

Could attentative staff play a role in better and fairer distribution?
Yes they can as in the flight preparation period, they could pre-seat Elites, block middle rows and ensure a fair distribution throughout the plane.
Do they do this?
At many stations yes, at others not.
Why not?
Because it costs time and effort.

For flights departing Amsterdam there is the thing of "missed connections".
A flight that's full can appear quite empty because passengers have been rebooked.

Tip: check the self service transfer machines at Schiphol Airport - that gives one a fair idea of available seats before departure, much more accurate than Internet Check-in and you can reseat until a certain period (I believe until the flight is closed for check-in).