Regarding passport control, the first part could perhaps read better as "owing to the United Kingdom not being a par of the Schengen Area, it chooses to apply additional security controls, and at the point of entry".  I have on occasions encountered no passport controls at Brussels and instead had an on-train check, or controls at Waterloo, or both.  I have been at Brussels at busy time and seen them waving past people with Lille tickets (meaning you could complete an international journey without hitting passport control).
Regarding scanning, this is associated with the Channel Tunnel, and the intergovernmental commission requirement to scan 25% of passengers (which Eurostar fulfill by scanning 100% of passengers).  Metal detectors/X-ray machines are also used before boarding high-speed services in Spain.
Regarding local trains.  I suspect you have in mind the impact of the Oresund Railway.  However, Calais is a tiny town and there is very little demand for through traffic.  The Channel Tunnel (and Eurotunnel) impose additional technical requirements (fire protection, train length, redundancy) for transit.  The transit fees are very high and extra equipment is needed to match paragraph height, electrical supply, signalling and radio in the Tunnel.  (See Eurotunnel Network Statement 2008).
Does that answer some of your points?  —