Most of the Eurostar cheap fares are for booking onlime. If you buy your Eurostar ticket online to colelct it at St Pancreas is the booking confirmation print out suffiecnt for your local station to sell you a Cheap CIV▸ ticket to London or do you need the full Eurostar ticket?
On South West Trains you require the full ticket following a chap fiddling the system a few years back, buying Eurostar tickets, printing confirmation, cancelling, etc.
Also, if you are travelling from, say Scotland or Cornwall, spending a night in London and getting the Eurostar the following day are you still allowed to buy the CIV ticket?
Assuming that your train is not reasonably possible to get to in the morning, yes.
Do you actually have to travel on the Eurostar for your domestic ticket to be valid or is it legal to use the Eurostar ticket merely to gain access to what is effectively an open return at the saver return price?
You do not have to travel on Eurostar. You must have a valid Eurostar ticket.
The CIV ticket is also available, I think, if you buy an international train/ferry ticket from Liverpool Street to Holland which would proably be cheaper than Eurostar.
London International CIV is available to St Pancras and Liverpool Street although I've been told that it's been removed in recent years for Liverpool Street due to the boat trains not really being boat trains at all and just being all stopper 321s.