Idle wondering as I'm looking at combi boilers and have little working
experience of such beasts...

Do the use a secondary plate exchanger running the sealed system HW
through the plate X or do some run the mains water (to the taps) through
the main exchanger in a separate water channel?

This is leading to:

Can you safely operate a combi with the mains water turned off -
assuming the radiator system is pressurised correctly - ie if you leave
the house for a couple of weeks in winter?

Never thought about it before - but if the HW is heated in the main
exchanger, and you turn the water off and lets assume a tap is on and
drains that side dry - would seem to be potentially bad for the boiler?