Are the Eurostar sets still in use with IZY, and will they continue to be once they return to service with ES next month?
Sadly not. IZY now uses PBA and PBKA sets out of the Thalys fleet.
Mind you, what would stop someone buying a ticket to Paris from getting off at Ashford? E.g they felt unwell or needed to dash home because of an emergency
It would be a very expensive way of travelling to Ashford!
I suspect also that if anyone got off a Paris or Bruxelles train at Ashford, or was even seen to be trying to do so by the platform staff, that the train would immediately be held, evacuated and thoroughly searched causing a huge delay so they probably wouldn't be the most popular person with the other passengers either...
Well, for a start you would technically be in France at that point, so I imagine that disembarkation would be strictly limited to genuine medical emergencies.
Passports now get stamped when entering Schengen from the UK so you'd have the issue of getting an entry stamp but no exit stamp, which can cause problems should you wish to visit the EU at a later date. It would also lead to a security alert, very specifically a bomb scare - the consequences from that would be severe, not only would you be extremely unpopular with your fellow passengers but you'd likely get into trouble for causing the security alert.
You wouldn't be able to decide to get off a plane for an "emergency at home" whilst it's queuing for takeoff, under a similar principle you can't get off Eurostar at Ebbsfleet or Ashford when heading towards Europe.
In theory Eurostar would be able to accept passengers boarding at Ebbsfleet if they wanted to (but not Ashford because of the shared arrivals/departures island platform there) heading to St Pancras, much like they accept passengers boarding at Lille for Brussels and at stations Lyon southwards heading towards Marseille, but for Ebbsfleet it's almost certainly much more hassle than it's worth which is why it's not done.
I once got off at Lille when I had a ticket to Brussels and nothing happened or was said. I was going to Bruggevand decided to go via a diffrent route than through Brussels for a change.
The issue that prevents early alighting at Ebbsfleet and Ashford from London is the Channel Tunnel. Once the train has passed through the tunnel, it's treated as a domestic train and passengers are free to get off at Lille (or if on an Amsterdam train, in Brussels and Rotterdam) if they wish, likewise if heading towards the UK, nothing to stop you from getting off early at Ashford or Ebbsfleet if you've got a ticket to St Pancras.
